The use case is that it will improve your search rankings, and giving you an argument to force your organisation and your ad-partners to make your site faster. It's effectively Google making up a tool they can force-feed big publishers to get them to make websites that perform less horribly, which makes the top results of Google search on mobile more useful and leaves more traffic to look at more pages and more Google ads.
There is no reason why a normal website can't be as fast or faster than an AMP one, but AMP gives the framework to get organisations that otherwise wouldn't care enough to do so.
This is the exact strategy Microsoft used for lock in way back when. You could use ASP and other MS technologies and they even worked well. Until you wanted to exit their walled garden - then they were a real pain to integrate with anything usual.
Isn't AMP available as an open-source library? I don't see the lock-in.
Besides, Google has announced that search rankings will be affected by speed, not by what library you use. So, if your site is already optimal (doesn't benefit from AMP) or you want to use some competing project that also does a great job, you won't be penalised.
There is no reason why a normal website can't be as fast or faster than an AMP one, but AMP gives the framework to get organisations that otherwise wouldn't care enough to do so.