A Search Engine Spider can find your site a variety of different ways. Often, it visits your site regularly once it has a copy of it in its own index, but it can also find your site by following links from other sites to you. Once you stop getting new links to your site, what is the incentive to have your site crawled? Fresh Content. If Google visits your site over a period of time and sees that your site's content has changed, it makes a note to visit your site more often so that it can re-index that new content and make it available in its Search Engine.

There's some speculation as to how much content has to change, but the popular consensus is that you need to have 1,000 bytes of information different on your page for Google to register it as a "Change of  content." This usually equates to between 200 and 300 words. But why do we want Google to crawl our site often?

When your site is new, when Google visits your site, it usually only crawls the first few pages and indexes only the home page. So, by causing Google to revisit your site often, it will crawl deeper each time. Before you know it, your entire site will be in the Search Engine Index. (Either way, Google will eventually get around to your entire site) But if Google knows your site is updated often, our friend the Googlebot will return more often which ultimately helps in increasing Google page ranking and gets your site ranked more highly more quickly.

When you're experimenting with your page optimisation, you want to see the results of your changes quickly. If you have the Googlebot visiting your site more often, it's possible that you can see the results of your changes between the major updates faster.

Customers like new content! Give your customers an excuse to visit your site!  Provide them with information that changes often enough so that they return just to see "what's happening." Sometimes the "popular" SEO advice is OK too - the common advice to "write lots of good content" actually can make a positive difference - as long as you build all the additional content pages correctly.

Whatever you do to generate the new content, the fact of the matter is that changing the content on your site, especially when it's new to the search engines, can go a long way to getting your site indexed quickly and thoroughly.