According to U.S. comScore Media Metrix, 185 million Americans spend an average of 29 hours online per person per month. Furthermore, an average of 22 searches are performed per user a month.
In 2007, Forrester Research tells us that the Internet (17%) is the third most used medium behind radio (19%) and watching TV (37%). I would be willing to bet that in 2008, the internet has moved into the second and is closing the gap quickly on watching TV.
These statistics mean your business should be (if you haven't started already) transitioning your advertising dollars away from traditional mediums (newspapers, magazines, billboards, radio, etc.) to online.
It has been said that "half of my advertising budget is being wasted, I just don't know which half". Spending valuable dollars in non measurable avenues such as magazines and billboards holds the above statement true. The only data you receive by these modes of advertising is the magazine's circulation or the amount of drive-by's past a billboard on a daily basis.
But are these people reading the magazines or driving past your billboard your customers? There is no way to really tell, you just hope your dollars being spent are getting in front of enough people, that eventually someone will contact you who needs your products or services.
Utilizing SEO allows you to spend money specifically targeted to those people actively looking for the products and/or services your company provides. You can't find a more targeted form of marketing than that.
There is some value of spending advertising dollars offline. However, the message you should be relaying is one that will attract those back to your website. Once you get a user to your website, you can tailor your message specifically to what that person is searching for. Search engine optimization gives you the opportunity to have a one-on-one interaction with an online user (185 million Americans!).
Be smart about marketing your business. Get in front of where the majority of your customers are (the internet). Don't overlook your website as the impetous of your sales efforts. The success of your business should begin and end with your website.