Here's something simpler from a non-developer, average-consumer point of view. I recently began taking BART to work daily (new job). For those who don't know, BART is Bay Area's subway system, and (at least on the east bay side) cell reception is notoriously spotty.
When I'm on the train, which includes 2 hours of my day everyday (unfortunately), I'd be browsing on say Facebook, and look at links that my friends post. Instant articles almost always load successfully (and quickly) and external links to actual sites almost always fails to load or loads insanely slowly.
Yes, when you're at home or in the city with good mobile reception, these things make no sense and you'd rather hit the original site directly. Give them their ad revenue, etc. to support them, right. But for the average consumers who actually have problems like slow internet (like the average joe who rides public transportation and wants to read on their phone), things like AMP and Instant Articles actually help. I can only imagine outside of silicon valley (where I live), how much more significant of a problem slow internet/slow mobile data actually is.
P.S. I don't work at Google or Facebook, and I know this sounds like propaganda, not to mention this is exactly what they would like to tell you as the "selling points" of these features, in order to continue building their walled garden empires. Fully aware of it, but I did want to bring up why they exist and why I even actually like them.