The problem this solves

A page's address is what every link to it points at. Search results, old newsletters, a post somebody wrote about you three years ago, a bookmark, a printed card. Rename the page and every one of those stops working on the same afternoon — and nothing tells you, because the people who follow them land on "page not found" and leave.

A redirect is how you keep them. It says "that address moved here", and a permanent one also tells search engines to move what the old address had earned over to the new one.