>>96964632
For human shaped mechs, the best use would simply scaling up the versatility of a human. This assumes situations where the use of hands provides an advantage.
My favorite mechs fall into the "chick legged"type; think the AT-ST from Star Wars or the Locust from Battletech. In theory, these are mobility mechs. The idea is that they would have great speed over difficult terrain. In my mind, this would give you a lot of the advantages of helicopters with out the noise and a lower power consumption while sacrificing actual flying and altitude.
Another type of mech I don't see often that I love is the crab mech. This is essentially a replacement for tanks and other tracked vehicles by giving them rows of legs rather than tracks. They could have heavily armored legs or they could tuck their legs under themselves when hunkering down. Two possible advantages over normal tank would be being able to take on even more rough terrain and possibly climbing steeper slopes and being able to move in any direction without rotating the body.
Just my thoughts.