100% accessibility is an unobtainable ideal — you will always come across some kind of edge case that results in a certain user finding certain content difficult to use — but you should do as much as you can.
To show that you care and have thought about accessibility, publish an accessibility statement on your site that details what your policy is toward accessibility, and what steps you have taken toward making the site accessible.
Consider accessibility from the start of a project, and test early and often. Just like any other bug, an accessibility problem becomes more expensive to fix the later it is discovered.
Accessibility is the right thing to do. Providing accessible sites is part of the law in some countries, which can open up some significant markets that otherwise would not be able to use your services or buy your products.