Amendment can be supported/voted?

I am moving here a debate that I opened in github (4 days ago) and no one has answered yet. I don't understand why the amendment can be supported/voted. We vote proposals, no amendments. In my point of view, I can't find reasons to do it. Moreover, it's going to be a problem. 1) I can vote an amendment to amend a proposal because I think that it will improve the proposal. But if this amend became a proposal, maybe I don't agree with that. When became a proposal, it will keep supports from it was an amend? 2) At least, is the author who decided if accept an amendment . So, why we need to vote? If the community wants that amendment, they can endorse as a demonstration of its agreement. Can you explain me the reason? Thanks!