Because you may want not to spend a perk point on the unbreakable perk OR on the master lockpicking perk. You don't have to choose both.
You pick Light Armor so you can run faster, move quieter, but can't take as much damage.
You pick Heavy Armor so you can take much more damage, but you can't run faster or move quieter.
I take a Perk in Heavy Armor that makes Heavy Armor Weightless, so I can run fine and not be burdened.
What's the point of ever picking Light Armor if I can make Heavy Armor just as light as Light Armor with the same, or better, protection?
The perk that makes heavy armor weightless can only be obtained when you get 70 in heavy armor. It takes a while to do so. During that time, you could be using light armor, which means you'd probably be able to carry more loot and get more money for your loot.




