Next time you're flying across the pond, try coding (without bothering to subscribe to onboard wifi).
Also, it can be useful in places where steady internet is sketchy, which is a lot of places.
The folks at 100 rabbits [1] would be happy.
And regardless, it seems like offline web API documentation would make more sense as a one-time purchase? It's not like the web is rapidly evolving at all times, with major updates being released annually. It's a good chunk of years before enough browsers are updated to support new APIs, so if you grabbed the current docs you'd probably be able to work with that for a while.
It's much more useful for specific API docs rather than general "how do I do X" stuff but it's very nice. Helps that you don't have to deal with scrolling through stuff like web3schools.
Stingy me would just wget download the entire documentation if I anticipate that I might go offline.
Can't say I'll ever be okay with paying for documentation; I don't want that to catch on.
But even when I have an internet connection, offline versions of most of these sorts of resources load considerably faster, mostly because I’m in Australia and these resources are normally hosted from America. My experience is that Americans that have always been in America and then travel to the other side of the world are consistently surprised at how slow the internet is away from the USA—and it’s all about latency, not bandwidth.
But I am compelled to admit that the performance angle is fairly negligible for MDN: it’s one of the extraordinarily rare sites that actually loads fast, with nearby TLS termination and content distribution and evidently nothing too outrageous in their coding so that it can consistently load to completion on my admittedly fast laptop from a cold cache and no open connections in well under two second, regularly under one.