From the Apache docs
The server will only follow symbolic links for which the target file or directory is owned by the same user id as the link.
So, enabling this option (as opposed to
FollowSymLinks) prevents symbolic links being followed that might point to some critical parts of the system (where the owner of the link does not match the target file).
Do I need it? I have a plain HTML web site (though might go for Joomla one day).
If you have a plain HTML site and no fancy URLs then probably not.
However, it is required by mod_rewrite, which is required for URL rewriting. (And is required by Joomla if you enable "pretty" URLs - ie. URL rewriting.)
Some sites specify that it replaces the
SymLinksIfOwnerMatch is a more restriction option. The two are mutually exclusive.