Google updates its relevance algorithm from time to time to ensure that it finds the most relevant searches for your keywords. Once this update is done it is always a good idea to resubmit your site, however, Google do not make it common knowledge when such an update is made. So I would recommend resubmitting to Google whenever you change any of the main content of your page.
Now I know you may have visited other SEO websites, the majority of which will say that there is no need to submit your site to Google at all, as Google sends its own spiders/robots out and will find your site regardless. However, this is where I have to say that I don't agree with that idea at all! Firstly, Google are one of the leaders, if not THE leader in search engine technology. So why would they offer the opportuntiy for webmasters to manually submit thier site, as proved here: Google Submit
Although I do agree with other SEO sites in the fact that Google will spider your site eventually, there is no harm in ensuring that your website is listed for the next update.
Sometimes you will notice that your site may no longer be listed within the index, this is usually because the algorithm has changed and your site no longer meets the correct requirements, so now you will have to update the content and resubmit your site. Constantly keep an eye on your site listing within Google, it is always a very good habit to get into.
Also ensure that you are listed within the relevant directories on Google. Once your site has been accepted into a Google directory (which is associated 'in a way' by dmoz.org, contrary to current belief. A great article is here) you can then submit relevant pages within your website to other directory categories. Now that your site has been added to a specific directory, you will not have to resubmit it again.