Small businesses commonly use website builder tools in order to create a professional website quickly and cheaply, without having to become expert Web designers, or learn about Web hosting.
A good website builder can provide built-in SEO (Search Engine Optimization) features that help grow a business quickly by driving plenty of organic search traffic from Google.
Website builders (the good ones) also continuously develop their platform to take advantage of new technologies and keep up to speed with new developments - like the current massive uptake in mobile technologies.