@Kelheor ah longer studies are important too, but these fast ones really help with comics/animation work.
@Iridescent_Bismuth I suppose I have 2 answers to that question.
One is to just make it a habit really. Even if its just 5 minutes everyday, it will do wonders over the long run. And over time, you'll develop a routine to the point where not doing it at all just feels wrong, even if you have other work to do.
Second is the one you kinda already mentioned. Ideally, your studies should be related to the comic/creative work you're already doing. Your studies should directly lead to your comic/creative work.
So for example, I'm doing some illustrations based off a moth/lichen inspired fairy character for a mentorship. Hence the moth animation study above, and I have a bunch of other studies involving lichen and moths and the like. So now that I'm working on the illustrations, even though I still have references pulled up just in case, most of the information is already in my head and I'm free to just draw/paint.
Its more of a struggle (and more time-consuming), I think, to over rely on references and figure out the hard parts as you go in the middle of an illustration, rather than just do some studies beforehand so you're not going into it blind. Even if it's just some really quick sketches that no will ever see, its still helpful to do them.
The reason most of my studies involve people, is because that's mostly what I'm drawing for my comic. Heck, I'm doing a NSFW comic and did a bunch of studies for that but nobody will ever see those lol. The idea is these studies are for you and your creative work, not for the public. So even if they're bad or lazy or sloppy, it does not matter.