How important is having a database optimized. Doing some "basic" optimizations like clearing the cache, runing the Drupal cron and a couple of things more, your database can change from having 4 GB to just around 200MB... incredible? Not at all, the importance of maintenance tasks :-)
Which tasks? Mainly:
Clean / truncate the sessions table. This table can grow up to several millions of registers.
Carefull with the spammers. Your comment table can grow up to similar amounts if too many spammers take a look at you. If the problem is big, simply delete with phpmyadmin:
delete from comments where status = '1'; // unpublished comments
delete from comments where status = '0'; // all published comments