So, aside from the many other powerful arguments for making your sites and your documents inclusive (doing the right thing, leaving no one behind, broadening reach, attracting the best personnel, fulfilling corporate social responsibility goals), it’s simply good business.
Structuring a site in a way that makes its content perceivable and understandable to people with substantial disabilities will also result in Google finding your content and ranking it higher.
For example, the same alternative text we add to images so that a screen reader can describe the content to someone who cannot see it, also is used by search engines to index that same content. The rigour we apply to heading levels to structure an accessible web page also suggests to a search engine the priority and context of our content.
Design is about making things work, often in an intriguing and delightful way. An accessible design is about making things work for everybody.
Here in Ontario, we live in the first jurisdiction in the world to legally mandate web accessibility not just for government sites, but for business sites, which is great for social justice. But considering how accessibility yields better ethical SEO (search engine optimization) it will prove beneficial for economic competitiveness as well.
So while the accessibility standards speak of how to accommodate all users, far better strategically is to delight all users…and communicators. So go ahead: delight Google. When we design for the extremes, everybody benefits.