Welcome to the Q&A Forum

I think it depends on where the website is - in the UK, it is a requirement to make sure that disabled people have the same or as similar as possible experience online as anyone else. Alt tags make it easier for the blind (amongst other groups of people) to understand the context of a website - a picture tells a thousand words and all that! I personally think its worth doing (though there are some missing on my site for various reasons) as even if you don't believe any of your visitors are using screen readers I bet some of them actually are. In the late 90s/early 00s the well known British supermarket, Tesco redeveloped their website to be accessible (not just Alt tags), and saw a staggering increase in sales and conversions as a result. I can't remember the exact figure but it was pretty impressive. So I guess I'm saying there are other reasons besides SEO and legal to implement accessibility features into your site - fundamental business reasons! Making more money. (though of course the cost must be weighed up against the benefit just like anything else).