Why do I need to wait to change my username again, each time I change it?
Every time you change your username, the waiting time until you can change again increases. We warn you about that every time you are about to change it.
Unfortunately, if we allowed users to change their username frequently, they would use this benefit to attack users and do other things that would be against our terms of service.
Usernames are not meant to be changed frequently, and this is why users have to wait every time they change it. Other big social networks also don't allow users to change their name frequently (or at all).
Changing username will keep your previous username reserved for a while (meaning that no one else can use it straight away), but also causes confusion to users who knew you with the previous username.