That's a good question. In fact, it's one of the most important and most frequently asked questions of the digital business age.
Yes, if you have a business, you should have a website. Period. No question. Without a doubt.
The point to be made here is that you should at the very least have a presence on the web so that customers, potential employees, business partners and perhaps even investors can quickly and easily find out more about your business and the products or services you have to offer.
That said, it's not enough that you just have a website. You must have a professional-looking site if you want to be taken seriously. Since many consumers now search for information online prior to making a purchase at a brick-and-mortar store, your site may be the first chance you have at making a good impression on a potential buyer. If your site looks like it was designed by a barrel of colorblind monkeys, your chance at making a good first impression will be lost.
One of the great things about the internet is that it has leveled the playing field when it comes to competing with the big boys. As mentioned, you have one shot at making a good first impression. With a well-designed site, your little operation can project the image and professionalism of a much larger company. The inverse is also true. I've seen many big company websites that were so badly designed and hard to navigate that they completely lacked professionalism and credibility. Good for you, too bad for them.
When it comes to benefiting from a website, size does not matter. If you're a one-man show or a 10,000-employee corporate giant; if you don't have a website, you're losing business to other companies that do.
Your website is an important part of your business. Make sure you treat it as such.