At the first AMP conference, one of the panelists, Jeremy Keith, was asked why he implemented AMP on his site. He didn't know. After some consideration, he said it was another distribution format, like RSS, so why not?
For some, it's about reach. AMP can be thought of as another distribution format, as Keith pointed out. To maximize your reach, you should embrace as many distribution channels as you can. As noted, some of the biggest search engines in the world, including Google, Bing, and Baidu and Sogou, are indexing and surfacing AMP pages in their search results. That's an audience of billions. And if your content is featured in the AMP carousels, that's even more eyeballs for you.
For others, it's about performance. Malte Ubl reported that the Guardian's regular mobile website was faster than its AMP page. So why did the Guardian implement AMP? Despite having an already fast site, there were still performance gains to be made via the AMP Cache and its instant pre-rendering.
If your competitors are using AMP, and are featured in the AMP carousel, then they have a competitive edge on you, and it could be worth your while investigating it.
Ultimately, if your site is not performing as well as it could be and this is affecting user engagement or revenue, then you should address this. AMP is about improving performance, so it could be part of your solution. Even if you have a hand-crafted super-fast site, like the Guardian, you might still have something to gain by adopting AMP.