What is the point to level up regular units ? Can someone list benefits ?

I am new to FE. I see that some units have traits like +something per level but there are plenty of units with triaits written in stone like "Dodge I" and after getting ten and more levels it doesnt upgrade to "Dodge II".

I was lucky to convert on my side a giant spider with Daze abillity and summon spiders. I did some leveling with it and he didnt get new abillities - only HP gain and maybe some accuracy.