Why would I want to limit the number of posts in the XML sitemap?

Hello. I looked over the Simple Sitemaps for Multisite usage, and it said that to edit the number of items indexed I could change the line:

var $totalposts = 25; // Number of posts to display

Why would I want to limit the number of posts indexed in the first place? I would want my site to have all the posts indexed by Google. Can someone please clarify? Thanks in advance.