Search Engines friendly urls are very important to have when developing a dynamic content site that uses long query strings. For example, most people would consider the second url more friendlier than the first url:

Query String url:
http://www.example.com/widgets.php?cat=blue&page=1

mod_rewrite url:
http://www.example.com/widgets/blue/page1.php

You can see that the second url looks much friendlier, easier to remember, and easier to write down.

RewriteEngine On
RewriteRule ^widgets/(.*)/page(.*)\.php /widgets.php?cat=$1&page=$2 [L]