For starters, setting up an account on one of these sites is very easy to do, and once a user is done the page they come away with is essentially a WordPress blog all their own. Or so it seems - in fact, if you are running this program on your own server and domain, these mini-blogs and site content will turn your visitors into content generators for you. If, for example, you have 50 people who are very interested in what your company sells, and you gain them as members, they will write on occaision in their blog, and those blog updates are fresh content for Google. If the posts are particularly good, people will link to them, and all that link juice will come back to improve the rankings on your site.
In a way, it is a brilliant way to get content developers for your site who will work for free.