A website allows for a much more robust experience for visitors than most other platforms. This means that you can be more efficient at providing visitors the information that they’re looking for, as well as the opportunity to create web applications for more value to your visitors.
Websites also allow you more control over your branding. While sites like Facebook allow some customization, it’s still inside Facebook’s shell. Companies are able to control all aspects of a design with a website.
Metrics and measurements to know how well your digital tactics are working are much easier to get via a website. Facebook and Twitter do provide some metrics (as well as some provided by third-parties), but the metrics are not as business oriented, making it more difficult to measure specific tactics and promotions.
If your web site looks professional, your potential clients will think you’re a professional who has enough clients and enough income to have a site built for you. If potential clients visit your web site and it looks half-assed and home-built that’s how they’ll perceive you.