Bruisers

so they can fight tanks

I sometimes laugh when people complain about Talon and Zed being able to get a lot of lethality, but when Garen gets Cleaver + E = 55% armor reduction + all the lethality, it's suddenly ok. I recently played full AD Garen, this thing is just unstopabble. W makes him really tanky, along with Cleaver, for the amounts of AD he gets and then just one shots everything.

Because most bruisers can't afford to build a lot of penetration, so instead they're given either free pen or hybrid damage of some sort. Some bruisers have both. This allows them to remain a damage threat late game, when their tankiness isn't going to amount to much against ADCs and mages with Void Staff/Last Whisper.