At the most basic level, a faster site will do two things: save money and increase revenue. Since those tend to be the two things that boost any company’s bottom line, page speed is increasingly becoming a concern for businesses, especially now that Google’s counting it toward search rankings.
It’s true that to be successful now, your site has to be fast. People just don’t have the patience to wait for a site to load. The basic attention limits have been the same since Robert Miller wrote his 1968 paper: Anything under 0.1 seconds seems to happen instantaneously. Anything that takes less than a second allows users to keep their trains of thought and doesn’t require special feedback. Anything over 10 seconds, and users have pretty much forgotten all about your site or app.