Adding text to the alt attribute of an image is a great way to help vision-impaired users (and Google) understand what the image is about. However, removing it can be ok in some circumstances. Sort of.
You can learn more about this from the full post over at Search Engine Roundtable.
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The main purpose of using the alt attribute on images is to help vision-impaired users (who often use screen readers) to better understand your site. With a well-built site, though, you might be making things worse for them, if the text near an image is descriptive of the image, and then the alt text is also descriptive of the image and it reads things to them essentially twice.
It’s still best to leave the alt text in place, but the real answer is a bit more nuanced. Here is what Google’s Gary Illyes said about this:
“In my opinion if the text around the image is relevant, leaving the alt attributes blank is sorta ok. Search engines would absolutely have less data from where they can infer what the image is about so at least in some cases that might have significant impact on Google Images traffic.”
I would think the cases where the alt text adds confusion for screen readers would be pretty rare, so you’re best to continue to add alt text to your images in virtually every case. Also, while this is an important piece for SEO, you should always respect the reason why alt attributes exist in the first place and keep your text informative and descriptive, rather than stuffed full of keywords.