Its the digital age, people use the internet for everything. If you make a call to someone and you mention you have a website, there is a high possibility that they will check it out. It also makes you seem just a little more professional then the guy next door without one. Printing it on your business card is great for a customer. Your website is the printed version of your sales pitch. You can talk until your blue in the face extolling the virtues of what you do, but if its not in print and theirs no pictures or proof, its harder to believe.
Also, since your website is a part of your sales pitch, dont half-arse it. Things like cutting corners will show, and it reflects right back on you and your business. You wouldn't show up drunk (I hope) to a prospective customer, so why would you act the same way towards your other advertising outlets.
On that note, don't use one of those free domains where you can post a site for free, like yahoo or such, it is very unprofessional and looks cheap.