What advantage's do Unit Experience points offer??

every 3 exp a unit gains they will be credited with either +1 damage or +1 HP in alternation
So at 28 exp a ranged heavy unit will have 17 hp and 16 damg instead of 12 and 12.
at 8 it will be 13 and 13.

The more exp, the stronger your units get. Exp'd units also are the last to die in battle so if they survive they will continue to become more powerful.

Which might I add is another reason range units are the best. (Of course with armor to protect them)

Because, no matter how much exp a armor or damage unit gets it will NEVER gain +1 range. While I currently have range units with 20+ hp and do 20+ damage and it's only tick 500.