Who uses POSIX realtime signals and why?

First of all, note that Ben's answer is correct. As far as I can tell, the whole purpose of realtime signals in POSIX is as a realtime delivery mechanism for AIO, message queue notifications, timer expirations, and application-defined signals (both internal and inter-process).

With that said, signals in general are a really bad way to do things:

• Signal handlers are asynchronous, and unless you ensure they do not interrupt an async-signal-unsafe function, they can only use async-signal-safe functions, which severely limits what they can do.
• Signal handlers are global state. A library cannot use signals without a contract with the calling program regarding which signals it's allowed to use, whether it's allowed to make them syscall-interrupting, etc. And in general, global state is just A Bad Thing.
• If you use sigwait (or Linux signalfd extension) rather than signal handlers to process signals, they're no better than other IPC/notification mechanisms, and still potentially worse.

Asynchronous IO is much better accomplished by ignoring the ill-designed POSIX AIO API and just creating a thread to perform normal blocking IO and call pthread_cond_signal or sem_post when the operation finishes. Or, if you can afford a little bit of performance cost, you can even forward the just-read data back to yourself over a pipe or socketpair, and have the main thread process asynchronously-read regular files with select or poll just like you would sockets/pipes/ttys.

Asynchronous I/O.

Realtime signals are the mechanism for the kernel to inform your system when an I/O operation has completed.

struct aiocb makes the connection between an async I/O request and a signal number.

It's an old question, but still.

POSIX threads on Linux in glibc (NPTL) are implemented using two real time signals. They are hidden from user (by adjusting min/max number constants). All events where library call must be propagated to all threads (like setuid) are done via those: calling thread sends signal to all threads to apply the change, waits for acknowledgement and continues.