A prerequisite of reaching the widest possible audience on the web is making sure you have an accessible website. Having an accessible website means your content is useful and understandable to as many people in as many scenarios as possible.
According to the World Health Organization, 2.2 billion people have a vision impairment or blindness. The subset of these people who interact with the web may have a hard time navigating or reading your content. These people may opt for assistive technology, such as screen readers. Those with less severe impairments could benefit from a better design strategy.
Vision issues aside, your visitors may have problems using a mouse with precision due to illness or injury. There are those who simply prefer to navigate a web page with their keyboard.
If your website does not implement web accessibility best practices you are not maximizing engagement and not delivering the best user experience.