If an alias is wrong, undesired or no more useful, I simply delete it from .bashrc or .bash_alias and source ~/.bashrc or close and reopen my terminal.
"Why would I want to wash my hands if I can just take a shower"?
Oftentimes that is an impossible or an undesirable action. For instance, suppose you had a bunch of processes running in the background in the current shell which will die if you close it, or imagine you were working on a remote machine, so relaunching the session will require you to re-establish the connection, type in your credentials, and in some cases struggle with two-step authentification.
Also, if you are just "visiting" a system which you don't have a set-up environment in (to troubleshoot someone's problems, for instance), and you are not particularily fond of their idea of making ls into an alias for less for instance, it is so much easier to say unalias ls rather than argue with the user about re-launching the session and editing their configs, or sufferring the bindings you don't like.
P.S.
I simply delete it from .bashrc or .bash_alias and source ~/.bashrc
That will not rid you of the existing aliases, unless you do unalias -a first.