2/2
It pushes you to think about what you are sharing. Trying to write a tagline which is clear and concise, requires you to elaborate about the picture you are going to share. Writing an alt-text could be considered part of a learning process.
I'm aware that this could be a debatable statement. Maybe someone else said those things before me. I don't have papers that back me up what I'm saying. It's an opinion about my experience. And I hope it could be helpful in raising a (civil) discussion.