> You describe how web pages work, web pages are intended for human interactions
Exactly, yes! The first few sentences from Wikipedia...
"REST (Representational State Transfer) is a software architectural style that was created to describe the design and guide the development of the architecture for the World Wide Web. REST defines a set of constraints for how the architecture of a distributed, Internet-scale hypermedia system, such as the Web, should behave." -- [1]
If you are desiging a system for the Web, use REST. If you are designing a system where a native app (that you create) talks to a set of services on a back end (that you also create), then why conform to REST principles?
Most web apps today use APIs that return JSON and are called by JavaScript. Can you use REST for such services or does REST require a switch to HTML representation rendered by the server where each interaction returns new HTML page? How such HTML representation can even use PUT and DELETE verbs, as these are available only to JavaScript code? What If I design a system where API calls can be made both from the web and from a command line client or a library? Should I use two different architecture to cover both use cases?
> Most web apps today use APIs that return JSON and are called by JavaScript. Can you use REST for such services
You kind of could, but it's a bad idea. A core tenet of the REST architecture is that it supports a network of independent servers that provide different services (i.e. webpages) and users can connect to any of them with a generic client (i.e. a web browser). If your mission is to build a specialized API for a specialized client app (a JS web app in your example), then using REST just adds complexity for no reason.
For example, you could define a new content-type application/restaurantmenu+json and build a JS client that renders the content-type like a restaurant's homepage. Then you could use your restaurant browser JS client to view any restaurant's menu in a pretty UI... except your own restaurant's server is the only one that delivers application/restaurantmenu+json, so your client is only usable on your own page and you did a whole lot of additional work for no reason.
> does REST require a switch to HTML representation ... How such HTML representation can even use PUT and DELETE verbs
Fielding's REST is really just an abstract idea about how to build networks of services. It doesn't require using HTTP(S) or HTML, but it so happens that the most significant example of REST (the WWW) is built on HTTPS and HTML.
As in the previous example, you could build a REST app that uses HTTP and application/restaurantmenu+json instead of HTML. This representation could direct the client to use PUT and DELETE verbs if you like, even though these aren't a thing in HTML.