Q: Why would you need to do this?

I've never had to do it but if search isn't finding things that it should, reindexing may help.

Typically you should not have to do this - but there are occasions where corruption can occur in the spotlight indexing mechanism - with the result being strange or inaccurate behavior in spotlight search and possibly in Apps.  Another effect is an observation of spotlight continuously indicating that it is indexing when it should not be.  There is no harm in forcing a reindexing to occur - other than it may slow down your Mac for a while.  An OS X upgrade or an unexpected shutdown could cause spotlight corruption - necessitating a forced reindexing.  I have personally never had to do this on any of my Macs.  If somebody is having to force Spotlight to reindex on a frequent basis - then it is probably a good indication of an underlying hardware problem.

~Scott