Aside from attempting to compliant for compliance’s sake, having an ADA accessible website can improve the overall user experience and site traffic. An ADA compliant site can increase your target audience (by making it easy for those with disabilities to navigate around), improve your SEO efforts (by helping search engines to more easily crawl your pages and content), and help your overall reputation.
The words in the tag should be more than a description. They should provide a text equivalent of the image. In other words, the tag should include the same meaningful information that other users obtain by looking at the image. In the example of the mayor’s picture, adding an “alt” tag with the words “Photograph of Mayor Jane Smith” provides a meaningful description.