According to the CDC, 1 in 4 US adults (approximately 61 million) have a disability. Oftentimes, members of this group will encounter a website that is difficult or impossible to use. A website that lacks accessibility makes it that much harder for disabled adults to purchase products, sign up for services, newsletters, etc.
Aside from people with disabilities, web designs can also affect those with temporary impairments or those of an aging population. Collectively speaking, this population makes up about 20% of the population. This means that your business is potentially missing out on a large group of consumers.
While there are no mandatory laws for businesses to have accessible websites, ADA lawsuits are far from frivolous and businesses are still finding themselves sued over their lack of compliance (visit this page for examples of cases).