Alt text: it's supposed to clue people in.
Imagine reading out your alt text to a friend.
Would they then be able to join in the conversation, on equal terms with someone who'd seen the original picture? Or did you leave out a crucial bit?
It isn't only for blind people. It might just be that someone's server is running slow that day and the pic hasn't arrived. But same principle either way: loop people in, if they happen to not be able to see the pic just then.
