I assume he is doing to he is trying to delete the buildings in order to make a better base or somehow make room for more important buildings instead of cutting down the forest which can be time consuming and expensive.
I believe the erase mode is just to be able to pick up one structure at a time instead of picking up all of them. For example, if you are creating a map and in the middle of it you accidentally do something wrong you want to erase one thing instead of doing it all over you go to erase mode. There is no way of deleting a building that you have already built.
My advice if you need more space just make more space.