Templates you create remain in the system forever unless you decide to get rid of them. One way to permanently get rid of a template is to delete it and it's gone for ever with no possibility of recovery. Another alternative is to Archive them. Archiving makes the template sort of invisible EXCEPT when you explicitly want to see them.
Thus, if you are 100% sure you will never need a template again in future, go ahead and delete it otherwise you are better off Archiving it.
Archived templates can be Unarchived but Deleted template can't be Undeleted.
