"Or are the Bearvaults meant more for areas without forest/trees?"
Bear canisters are intended for areas where there are bears, and especially for areas where the bears have gotten pretty smart about stealing food from the backpackers. In some areas, the trees are few and far between, and the ones that you find are not very suitable for hanging a bear bag. Near timberline, some trees are pretty short, and their limbs don't stick out far from the tree trunks.
Black bears have hooked claws, so they can climb a tree pretty fast.
The more that black bears steal the backpacker food, the more likely that they will continue to torment future backpackers, and that gets a vicious cycle started. You can't have a bear intimidating backpackers to get their food.
Some of the national parks found that when backpackers use bear canisters and bear lockers, that the cycle is broken. Black bears go back to eating their natural diet, and things get back to normal.
–B.G.–