Read the introduction and conclusion for usecases. Self-hosted FaaS gives a "serverless" hands-off experience, whilst making packaging and operating flask, Go, Node, Express, CSharp, etc, etc trivial. Try it, see what you think.

You don't have to keep it running in memory all the time.

If you have a small device with limited resources, maybe you'd rather have it be ready to potentially do many different things for you infrequently, as opposed to doing fewer things continuously all the time.

Isn't that the OS job?

It was my understanding that the kernel would put idle process to sleep, and even put the memory it uses from ram to disk and wake it up if there is some input to it.

How would the OS know that web server process is "idle" vs "ready to accept requests"?

Not sure, socket binding ?

I was taught in school "The job of an operating system is to share resources" so yes.

Ok, that's a fair point. So maybe some serverless tech can be more specialised and thus better than the Linux kernel at managing, effectively, virtual memory.

If everything is written in-house I still think any API would be simpler but I guess this way some functions can be supplied by vendors/open-source which currently cannot be composed into an e.g. flask API. So it would be nice to have a way of composing APIs from simple functions supplied in packages for this use. I guess that is what serverless is.