Obviously, if you keep updating your blog it will help draw in more traffic. You are putting more and more content out there, which means it's increasingly likely that your blog posts will appear in search engine results. Even if you don't optimize your posts for any keywords at all, merely keeping at it will result in the blog ranking for a few long tail searches eventually.
But there's another benefit if your blog is attached to a main site. Regular (and preferably frequent) posting seems to confer search engine benefits on the TLD as well.
I have seen this occur on one of my websites recently. I've been posting to the site's blog almost daily for a couple of weeks now and I'm getting a noticeably greater number of search engine hits to the main domain. (I could be mistaken, of course. There could be some other reason I'm seeing this improved traffic, and I just don't know what it is. But I don't think so.)
The benefit makes sense. Of course Google would prefer to show live, active sites in its results. If you update your site, tweaking the content from time to time, then you are telling Google that it has a pulse. And if you add a blog and keep posting to it, then you are showing that it's alive and kicking.
So I think it's a good idea to look at your blog as a kind of winch that's attached to your main site. Every blog post is like a turn of the winch, gradually lifting the main domain a little bit higher in search engine results.