One of the metrics for Search Engine Optimization is the time it takes to load a page. With Google’s PageSpeed tool, you can see if your page is loading optimally. One of the ways to better your page loading is by leveraging browser cache. The article below explains it well. What I did was a little simpler. I basically just bypassed all the ExpiresByType and just put the ExpiresDefault in my .htaccess.
<IfModule mod_expires.c>
ExpiresActive On
ExpiresDefault "access 2 week"
</IfModule>
keywords: leverage browser caching
source: http://thomasgriffinmedia.com/blog/2010/11/how-to-leverage-browser-caching-in-wordpress-via-htaccess/