Well, it's a lot more important than it used to be! The days are long gone when a law firm could slap up a bare-bones website, with minimal functionality, and leave it untouched for years as it maintained its high placement in Google search rankings.
The search algorithm Google uses today is much more sophisticated than it once was, and one of the things that it tracks is the frequency with which your website is updated—and whether these are minimal changes (say, fixing typographical errors) or more substantive additions and overhauls.
There are two kinds of updating, one of which is more important than the other. You can pretty much leave any existing content on your site alone, especially if it's performing well, though you should take care to update it with links to your newer content when necessary (this sends a strong positive signal to Google).
What's most important is to constantly post new pieces of content to your site—blogs, frequently asked questions, news items pertaining to your practice areas, etc. The more well-written content your website has, and the more frequently this content is added, the better the “quality” signal you wind up sending to Google—and the result is that you will climb in search engine results, from which exalted position it will be harder for your competitors to dislodge you.
Do you need help deciding on the right mix of content for your website, and deciding how (and when) that content should be updated? Call the Internet marketing mavens at Great Legal Marketing (888-791-2150) so we can get you pointed in the right direction!