Google saves a copy of web page every time crawler visits a web page to check for new content or to check the relevance of a web site to match the search query of the user. If for some reason web page is not opening and user has put query, then google shows the cache content to the user, which is as per the last indexed report. Cache content is shown to the user with a header, that mentions that it may not be the most recent page and the query of the user is highlighted in that page.
yes Google cache is useful for SEO, If Google bot visits a web site and sees 304 ( “Not modified”) response and not “If modified”, then Google bot will not download the web page, which will reduce the bandwidth consumed on the web server. This primarily means that more number of time google does the indexing of your web site, actually means google is looking at it as a site carrying authority, which is good and is an important part of SEO.