table prefix at intallation/setup - yes or no, and why?

I searched for this in forum but found nothing specific:

During manual setup/install, at Database setup page, there is an entry field for Store Identifier (table prefix). Someone please explain in relatively simple terms why one would or should use a table prefix? (I also noticed that the Fantastico installer creates the table prefix zen_ by default). I may want to set up a "sister store" with a different name and design on another domain but share one db. Is this possible and would it be a reason to use table prefixes? My webhost allows unlimited MySQL db's so thats not an issue really. I'm clueless on this one. Thanks in advance for a quick bullet list of "table prefixes for dummies".

Purely as an identifier. No more, no less. So if you have no limit to the number of dbs, then you don't need to bother with it... and the usual recommendation is to dump the Fantastico install and download the latest version of ZC from Downloads and install that.