Real world example: Rose Oil
Please read these two articles https://en.wikipedia.org/wiki/Rose_oil and https://archive.org/details/b28059979/page/n299/mode/2up
To make Rose Oil you need a specific flower harvested at a specific time of year and a specific part of day (just before sunrise) to get a worthy ingredient.
Then you start processing it by boiling with a lot of water and distill it repeatedly. Even with a long and polished process you get an extract of only about 1:3000 (yes, just one of three thousand) of the flowers (not counting the spent water, fire, etc.) which is supposed to be able help allergies, if it comes into contact with the skin. After some more processing and dissolving there is some refined product, which (while wonderful) is not even magical.
The process has been basically the same for hundreds of years. And yes, the rest of used ingredients are just trash after the extraction, and not good for anything, maybe compost. All that matters is the 1:3000 part of roses, the rest is just totally wasted.
A lot of people work on getting some reasonable results from the harvest, as it can be done only once a year and the the field cannot be used for anything other.
Magic world discussion
Typical magic potion is done from a lot of special ingredients, usually by single witch, living far from people (for many reasons such as their safety, her safety, the waste she makes (and that is probably not only terrible, but also toxic and maybe even cursed), secrecy of her recipes, good place for harvesting, ...)
The recipe will probably will require some refined oils and other extracts (and not just one, but say 3-5-7-9 or something like that, depending on final product) — in specific combination, and used under specific circumstances, like under full moon (both for its magical effects and polarised light) and some others — to extract the real magic from very specific ingredients. And yes, those extracts have to be undiluted and they are wasted after the use.
But the result is real magic potion with specific effects.
Now, while the final extraction yields one small vial from like 5 liters of extract and a few really rare ingredients (say optimistically just 1:100 rate), and takes just few hours.
But try to count, how many specific flowers, mushrooms, roots etc. had a single person to harvest, and that probably each of the extracts is based on some other ingredients, which have to be harvested at another time of year in insane quantity and processed by a really long procedure.
No wonder, that nearly all the time there is a big cauldron, in which is something boiling. And that the real treasure (some bottles of different extracts placed in some convenient place, say in cellar, where is more consistent cold and humidity) is not visible to unknowing eyes, with the cauldron on display and flowers, roots, branches, mushrooms are drying everywhere possible.
The witch does not know what she will need after a half, one or two years, but she knows that it would take some insane time to process all those ingredients and that it could not be done in a few days, as some things are ripe in summer, some in winter, some under the full moon without clouds, others on stormy nights with a new moon etc. etc.
And with witchery, instead of modern chemistry, she is not really sure, what is necessary and what is just optional — she knows what worked for her grandma, she may have discovered something herself, but when there is always life at stake, there is just so much you can try to change in one step (and probably just lose a lot of Rose Oil and other extracts, so it would be really expensive, even if nobody dies from that).
So she'd rather stick to the rules and add one young living newt from the swamp, while maybe there are just needed some impurities from that swamp mud, which also contains some sulphur and arsenic in balanced amounts. Just how she could be sure, when there is working recipe and some lore, that it one time worked this way and so everybody in her family do it this way as precisely as possible.
Moreover as she processes all kind of dangerous, boiling, flammable and toxic substances, no wonder that her usual clothes are in a terrible state (they were new long ago, but if you know that nearly every time you work you damage them somehow, then you really do not use new clothes for work) and why she looks like she looks when regularly in contact with all kind of allergens, toxins, acids and such.
And it is also one more reason, why final extractions are done far away from human eyes — not only for tradition and protection of her secret recipes and avoiding risk, that somebody would like to see what she does and then accidentally knock her equipment down, resulting in total loss of much work and his life — but also that she does it usually naked or at least underdressed — yes tradition too, but also there is too big a risk that something from her clothes may fall into the mixture and destroy it totally.
So I always believed that potion making witches do a lot of work at the time they can and save a lot of extracts for those final mixtures somewhere hidden to be able use them in reasonable time, when they need some specific potion. And, as they also supply last resort healers for many villages around, that they have a relatively big haul of half-made healing potions ready, usually just boiling in the cauldron, as they can expect that somebody will come soon with some problem and then they could just take part of that base, add specific drugs and make some "medicine" in a relatively short time at the cost of the "medicine" expiring in days or max weeks and being unwieldy, but sufficient for "simple" healing now and here in time of days — while real potions which would save you in a dungeon after years of waiting are much more concentrated and expensive and harder to make.
On the other hand if they want their privacy until their help is really needed (and then add a big part of placebo, as it works too), they keep some strange looking things around, which are not threading them (bones in cauldron, strange colored things of unknown origin, but after distillation totally useless and mainly inert and such) but are quite impressive to the customer.
And if all work is done and nobody comes, well good broth with vegetables and mushrooms could be eaten as well instead of using it for diluting the real weak healing potion :)
You know, why they add a lot of something to the cauldron, when someone comes and asks for medicine — if there is just strong soup, it is good to use it as carrier for the real thing and make sure, that customer is not able to drink a week's worth of active substances (which he would do for sure, if just given small vial and told how many drops daily, but if it is one full spoon morning, one evening and say this long prayer before and after, the risk of overdosing is way smaller).