What does improving the Engine do for me?

I’ve read multiple guides so generally, I know what stuff does. Things like employee stats, good combinations, etc.
I’m not entirely clear as to what improving my Engine does. Naturally, I’ve gotten to the point that I can spam new Engines (mostly so I can farm Research points), but I’m wondering. Is there some percentage of improvement it does to my game quality/sales/reviews?
Speaking of percentages, what do those mean during the game phases? If something is at 35%, does my game suffer? Or does it not matter if it’s a combination of creation that doesn’t emphasize that particular category?

I’m not sure if my questions are coming across properly. Sorry about that.

After a while some features are more of a requirement (2D Graphics/3D Graphics v[something]) and for a sequel it is better to use a better engine.