Hey Amelia.
Yes you are absolutely correct, alt tags are for people, and should be used to that end. When I said it 'tells' Google what the images is about I was responding directly to Shawn's comment that ALT tags are for accessibility, not SEO...
Technically yes, but Google does use the alt text as a relevancy signal (at least as far as we know). And ranks your image (in Google images) based primarily on that information. It’s a part of SEO for better or worse. Besides, you can both improve your on-page AND give relevant and meaningful info to the user, i've never had to choose. If it came down to it I’d always prioritize the user.