The math is fairly straightforward for this one:

Each creature has a set of base stats. Let's say a level 1 creature starts with 15 of each stat. Each time it gains a level, it gains 20% of its base stats, which is 3 in this case.

When you use a pill, it gives +1 to a certain base stat. So let's say that same creature's base Luck becomes 16. Each time it gains a level, it will gain 3.2 luck instead. This effect is retroactive, of course.

This is a much bigger deal for normal creatures than Nether Creatures, and it's definitely not intended to be game-changing. Just a small bonus that stacks up even higher over time. I suspect that when I change the way pills are acquired, you'll have access to more of them though.