Because it gets you an immense amount of existing tools that you won't have to reimplement. You can get shells, compilers, and numerous utilities (eg cp, cat, tail, tar, zip, awk). Look at the list of what BusyBox includes to get an idea of the kind of functionality any system would need to get started. http://www.busybox.net/downloads/BusyBox.html
If you provide terminal emulation then you also get editors. If you implement ptrace then you get a debugger. If you provide networking then apps can display remotely (X).
Even if you are developing something unique for your operating system, using POSIX in that lets you perform some of the development and testing on other systems that already have working toolchains.
In short having POSIX saves a huge amount of time and effort. That doesn't preclude you from having other APIs around too. Don't underestimate the importance of having a functioning system while you replace or augment it with parts that are your unique value add.
Sure, but keep in mind that my ideal would also involve discarding most what a POSIX compatibility layer would get you. Why cleanroom a nifty kernel and then turn it into something that's almost exactly like what already exists? If you want Unix, you know where to find it, as the wag said.
The kernel doesn't have to support POSIX. You can do the emulation in user space. The only tricky part of POSIX is fork, but chances are you won't need that for compilers etc, just exec.
In any event don't confuse the journey (some POSIX compatibility in order to advantage of existing toolchains while building your new OS) for the destination (clean, elegant, new API, world changing, nifty) OS.
And if you are going to have command line tools in your OS, they will need an API and you may as well pick a useful subset of POSIX.
So at some point, you can just put the whole FOSS environment (Firefox, Gnome, Gimp, LibreOffice, etc) on top of your kernel. Porting all those programs is much more work than writing a POSIX compatibility layer.
If you provide terminal emulation then you also get editors. If you implement ptrace then you get a debugger. If you provide networking then apps can display remotely (X).
Even if you are developing something unique for your operating system, using POSIX in that lets you perform some of the development and testing on other systems that already have working toolchains.
In short having POSIX saves a huge amount of time and effort. That doesn't preclude you from having other APIs around too. Don't underestimate the importance of having a functioning system while you replace or augment it with parts that are your unique value add.