Simply put, because you want to make money.
No matter what your business is, you have the aim of bringing in money. As technology changes, so too do audiences. Ignoring these changes would cause difficulty in maintaining income in your business. Although the world of the internet and websites isn't particularly new, there is still some hesitancy from some businesses to take advantage of this platform. As time goes on, it seems like the world of websites is becoming more and more complicated. This is one reason for some of the reluctance to create websites by businesses.
However, you need a website if you want to keep selling to the modern market. The internet, at its core, is a great tool for information gathering for the general public. For most people, they will turn to their favourite online search engine to get the answer to find they need. These searches have been known to exceed 5.6 billion per day. That's a lot of people searching for a lot of information.
Why do I need a website?
If you don't have a website for your business, it is unlikely that anyone will find you when searching online. Whereas if you have a good website, with good Search Engine Optimisation (SEO), you'll have a much better chance of getting business out of these searches.
Even if your business is doing well now, you cannot guarantee you won't have a lull later on which you need to fill. A website is a great way of ensuring you can keep your turnover high during these times. From creating mailing lists for email marketing, to social media advertising directing clients to your site, a website is a great tool for bringing in business.
There's a lot more specific points for different industries, but the general message is this: Why do I need a website? Because you can't afford not to have one.