Depends on what you're updating the blog with
(i.e. the content of the blog posts). Posting a sentence or two of crap every day would be regular updates, but wouldn't do much of anything for the quality of your site.
It also depends on what you do to promote the blog. If you post quality blog content and participate regularly in the blogosphere so other bloggers become aware (in a positive way) about the existence and quality content of your blog -- such that those who write about related topics start to link to you and/or mention you in their blogs and/or ask you to write guest posts etc. -- then a blog can help.
The inbound links will bring link popularity to individual blog posts which can then pass that "juice" around to other pages on your site via internal navigation links.
The mentions and links may also bring traffic to your blog, which (if it's properly linked in with the rest of your site) can bring qualified visitor traffic to the rest of your site.
The point is, a blog is time consuming and hard work, both writing it and promoting it. Just adding a blog isn't going to do much for you one way or the other in and of itself, any more than any other kinds of web pages will.
You have to commit to putting in the time and effort to producing quality content and getting the word out, just as you would with any other website content.