What is the purpose of attaching a database?

I always see this option called attach database in SQL Server. Why would someone attach a database? Why not just restore a database from the .BAK file? I guess I don't see the purpose of it. What would be the result of attaching it? It's quicker than a restore, but what is the technical result? Can you still alter the database? Add data to it? The preferred method or not recommend it? I have seen https://dba.stackexchange.com/q/11177 but nothing in there explains the purpose of attaching or detaching. It basically doesn't even touch on it.