Bipedalism is actually not that rare among animals like us, ie primates. Lemurs and gibbons walk on two legs on the ground (though lemurs tend to skip more than walk). 20 species of gibbons and they all do this. For some tree climbers it just seems to be the most comfortable way to move around on the ground.

It seems to have been the same with our ancestors because full bipedalism was already present right at the start of human evolution with Australopithecus, which still had a brain not much larger than a chimp. So maybe bipedalism is not something we evolved for any particular reason, maybe it was just how we started out as a byproduct to how we moved in the trees, and when we started living on the ground more we had to work with it.