If your database is working good why you really have to do any maintenance on it. Are you OK just doing backups?
Provided you don't add / change / delete any data then "yes". But then you won't need any more than just one backup either ...
Indexes go out of shape - and need restructuring
Files grow, they need handling to remain optimum (e.g. de-fragment)
Files grow inadvertently - worth shrinking them back to normal-working-size - or be in a position to know the growth rate and therefore predict & plan for hardware expansion
As tables grow queries that are not correctly optimised will become apparent (they will slow down ) - they need optimising
Database might become corrupted (physical damage to the files), and early detection makes the difference between a straightforward sort-out and a nightmare scenario
and after all that Backups need restoring (somewhere different) just to prove that they CAN be restored.