Whenever I'm using mobile internet, alt text allows me to fully understand posts without having to use up a significant amount of mobile data just to load all of the images for posts I see.

And even when I'm not on a metered connection, it's still a great way to see an alternate angle of what an image means!

So many times I've seen a post which has an image that confuses me, and usually after reading the alt text it makes a lot more sense.