Is REST even designed to be used by a SaaS? Are there any examples of this? What’s all the jazz about it whilst it’s not even being used widely
Yes, REST is there for machines to communicate with each other and bla bla and bla.
What’s the use of it is no user-centric real-life software uses it where user commands the machine to communicate with another machine?
If this is the case, in which REST is there for APIs and SOLELY and ONLY web-services, what’s all the jazz about it?
If you look closer at the site in which you have just placed your question, you may notice it uses REST actively.
I am not sure whether I understand your question properly (it may be that either my, or your English is insufficient), but if you look at the modern sites, you may notice that in most of them the user interacts with JavaScript (one machine), which interacts with the server (another machine).
I guess that REST is considered so good idea, because it is simple to define, simple to implement, and once you get used to it, you can interact with any service in similar manner.
If your question is about something different, accept my apologies.