When your unit levels up, you can either heal it for free or get a strength bonus (have not encountered any other perks yet). However, when I choose to get latter, it does not show anywhere - unit stats stay the same, there's no perk icon or anything.
Do those actually work?
Edit: nevermind, they do actually show up when you mouse over XP bar. Also you seem to get XP when doing so, what's up with that?