In any modern story nowadays, shapeshifters always have an original form; Andalites from Animorphs or Mystique from X-men to name a few. It raises the question of why something that can change forms would even bother with an original form. Ignoring the unrealism of a creature evolving the ability to shapeshift, why would a shapeshifter bother having an original form?
A shapeshifter doesn't "choose" an original form. A shapeshifter's "original" is whatever form it started as - that's what "original" means. That form is whatever it was in before it gained/learned/discovered its ability to shapeshift.
In cases where maintaining a shape requires concentration and effort (Odo in Star Trek: Deep Space Nine), has a maximum time limit, or is triggered by an outside influence or external catalyst (classic werewolves), a shapeshifer may have a default form that they revert to when resting (Odo turns to a liquid) or in absence of the catalyst (werewolves become human).
Shapeshifters without such a restriction (Mystique from X-Men) or with a default form not conducive to interaction (Odo, again) may have a preferred form. As Cort explains, having such a form helps such an entity reinforce its sense of self. Sentient beings in most stories want to have some way to be relateable to other sentient beings; a preferred form helps you maintain those relationships, too.
In many stories, shapeshifters have a lot of difficult with their sense of self. In a world where a physical shape is often associated with an identity, they would continually be bombarded with interactions from everyone based on their physical appearance.
Retaining an original form would be one way to make sure you have an identity that you don't lose over the years. It's something solid and stable. (except for those rare circumstances where one shapeshifter emulates another, but its rare to find stories where they try to rob another shapeshifter's identity).
Reasons for why a shapeshifter would return to its original form:
- A shapeshifter cannot remain in a new form forever and must return to its original form in order to survive.
- Staying in another form requires a large amount of energy or concentration.
- A shapeshifter must stay in his original form in order to have access to the benefits of his identity, such as his bank account for example.
- A shapeshifter chooses to stay in its original form when it's around its childhood friends, to prevent confusion.
- The shapeshifter feels sentimental about its original body the way we feel sentimental about our childhood home or our favorite movie.
As a metaphor, I will use online screen names.
A person can potentially have as many screen names as they like, with a different one on every site, and potentially several for the same site (discouraged as this might be). So why is it that most people like to use the same screen name in multiple places?
People like to have an identity, something that others will recognize immediately. While there are exceptions, it is human nature to want a symbol to identify themselves, taking on other forms mainly for practical purposes.
Unless a shapeshifter is particularly antisocial, it stands to reason that they might feel the same way. This is also why a shapeshifter who does enjoy mixing it up somewhat might have a common 'mark' possessed by all their forms (for example, all of their forms have the same color skin or fur) so that they can be immediately recognized.
An antisocial shapeshifter will probably be different; in their case their shapeshifting may be an acquired power and they must expend energy to sustain an altered form. Shapeshifting species are prone to defaulting to an amorphous 'blob' form when resting because this shape takes the least amount of energy to maintain. Alternatively, they may have a form that they have more experience with and will generally stick to that one when another isn't more practical, simply because the familiar is comfortable.
Antisocial shapeshifters with capricious personalities and without physical limitations are the least likely to have an 'original' form. Bartimaeus from the Bartimaeus trilogy is a good example, although even he has a common thread between his many forms (he prefers to take the shape of desert animals when possible).
Supposing you had a shapeshifter for which any form would be equally fitting (ie. we are talking about preferred forms, not original ones), and living in a non-shapeshifter society, I see several reasons for them to have such identities:
Keeping the face people is used to. This is specially important when dealing with people unaware of your shapeshifting abilities, but it is still appropriate in general that people is able to associate your body with your identity.
Bureaucracy. Generating a background takes time and effort. Even though as a shapshifter you can change your appearance as much as you wish, you will probably have only a few identities with a driving license. Similarly for other tasks, such as opening a bank account or renting a house. You may be good at creating fake documents, but some will still require a more careful creation. And they won't be as as shape.
Related to the previous reasons, some identities will require a more detailed crafting: name, job, marital status, studies… Some identities will need the shapeshifter to remember lots of information not to contradict himself. Thus these can be considered as "permanent" identities even if only presented to a few people.
Sentimental reasons: Martian Manhunter adopted the identities of dead people (often killed) as a tribute to them.
Ignoring the unrealism of a creature evolving the ability to shapeshift,
Chameleons can change their skin color. Not shapeshifting, but a first step to that direction. As a sidenote : they have a default/resting/original color that is different for each chameleon species but mostly green or brown.
why would a shapeshifter bother having an original form?
Assuming that the shapeshifter species developed this ability during their evolution, the "original form" is something they inherited from their ancestors.
I think it depends on how the actual shapeshifting works.
In engineering there are three type of deformation:
Elastic Deformation, Plastic Deformation, and Fracture. I won't consider fracture.
In elastic deformation you apply force to an object to get it into a shape. Once the force is gone it will revert to its' original shape. Think about an elastic band. If your shapeshifter is an elastic deformer, then reverting to a natural form is what happens when you stop using the force keeping you in a particular form. Keeping up the shapeshift costs energy so to stop draining that they will revert. That's why they prefer their original form.
In plastic deformation you apply force to an object to permanently change its' shape. Think about: clay. If your shapeshifter is a plastic deformer, it probably won't revert to a natural state. Because keeping up the current form doesn't cost extra energy but the act of shapeshifting can cost a lot of energy. That's why they don't prefer an original form.