The benefits for users are obvious: they get the information they're looking for much faster, within a familiar interface and with less clicks.
The benefits for Google are also clear: they make the impression of making people's lives easier by connecting them with the information they're looking for more efficiently.
But what does the website owner, who provided the information, get in return?
If this is an event website, the highligher will show links to the event pages, and the user can just go there directly. I get it. But then again, if someone's just looking for an event date, or a place address, they won't even visit the website, because they can find that out on the SERP now. There will be no clicks/views on ads, and no visits reflected in the site stats. Or if it's a news site, AMP will show the text verison of an article, without the other UI elements such as related or featured posts.
Found Benefit of Google Highlighter when searching, but that's about Google Highlighter vs. Schema.