Single-purpose or embedded systems. Eliminating disks means one less thing to go wrong. Many persistent security threats cannot occur. If you want to deploy a software update, reboot the machine. No need to worry about remote updates/reboots/fails. Disk state is irrelevant if there are no disks.

I shipped a system like this many years ago (scoreboard displays). Most netboot schemes require the network to be up for the client machine to operate. If you run from RAM, like this system, you only need the network to boot the machine. It can then tolerate outages or run disconnected.

Any datacenter usage, where you save money on hard drives by running your OS in RAM and mounting the work storage over the network. This also provides faster recovery from errors, since you're not storing any state on the working node.

It's not. You save money (OPEX and CAPEX) using diskless boot.

For example debian in memory including everything you need on a server is just below 400M.

We builded a lot of datacenter services on PXE on the past years. It's always handy if you do require more resources than a single server can provide. It's like a container on steroids.

Any kind of thin client/zero client usage, where local storage is not required or allowed. Many years ago I used a similar solution for a finance company. The users just needed to get access to a browser to connect to a specific web service.

Ephemeral logs.