On Twitter, Google’s John Mueller confirmed that anchor texts and the texts in img alt attributes help Google to understand the content of a page.
When you create your pages and links, do not forget to include relevant words in the links and the image descriptions. Here’s John Mueller’s tweet:
@abondance_com @methode Yes, but anchor text (and image alt text) helps us quite a bit in understanding context, so I wouldn't leave it out if you can avoid it.
— John ☆.o(≧▽≦)o.☆ (@JohnMu) 15. Mai 2017