What environmental pressures could drive evolution of a creature that has the ability to float? How it floats isn't important.
If it has multiple stages to it's lifecycle and the final stage is fairly immobile then floating "spawn" would make sense. You'd also probably want to make it an R-strategy organism which produces many young of which only a few are expected to survive.
You could also make the environment quite hostile such that food and stability in any particular area is unpredictable.
You could also model it on jellyfish lifecycles only with floating in the air.
Floating isn't the best way to get around but it's cheap and easy: if you're producing a thousand offspring letting them float away is a good strategy.
Indeed a number of spider species use "ballooning" where they float away on para-shoots made of webs to reach new locations.
Float away to breed another day.
Floating isn't the most rapid, stealthy, or graceful of transportation methods. So it won't as likely be used for a predator, unless it's something like a floating jellyfish.
But being able to float on water or in the air has a significant advantage over a predator that can't fly or swim. So it would likely be developed by prey animals for the purpose of escape.
The need for surprise attacks.
Most humans (and, in many cases, animals) don't think in three dimensions. All of the people we interact with are, in general, at the same elevation we're at, moving at the level as us. When you walk down the street, you don't often look straight up.
Something that floats has a tremendous advantage, because it can simply descend on someone or something. For example, if an animal lives on the surface of the ocean and floats using an air bladder, it can quickly descend on potential prey from above. They might not notice it coming, if it moves fast enough.
Floating is easier than flying or swimming, and takes less energy.
Perhaps it wasn't a species of the animal kingdom that originally developed the ability to float, but the plant kingdom. A floating plant would have a significant advantage over its earthbound competitors, in that it occupies the "ultimate high ground", gets first chance at the incoming photons, and can cast deadly shade on the unfortunate specimens below.
After that, floating animals could either evolve from those plants, or perhaps use those plants to achieve buoyancy somehow, or maybe just develop their own floating method through parallel evolution. The first species of herbivores to develop the ability to float will have exclusive access to the entire habitat of the floating plants, and completely escape their lower-altitude predators, both of which are significant advantages over their ground-bound cousins. The first species of floating carnivores will have exclusive access to the floating herbivores, which is again a significant advantage over their heavier competitors.
As always, evolution is an arms race, and air supremacy is a powerful strategy.