If players (both male and female) want a female character to be recognizably female although she's wearing plate armour and just a few centimetres tall on the screen, boob cups are a way to achieve just that
Another way would be to have pink armour.
That wouldn't help for Death Knights, though.
In act 2 of Beyond Divinity you are tasked with stealing a box and bringing it back unopened. If you open the box, the Death Knight's armour turns pink for the rest of the game (video).
