Historically, the US Government. NIST mandated that only those operating systems should be used where existing applications can easily be ported to and fro, and POSIX compliant OS-s tick all the boxes. Windows NT got its POSIX subsystem for this reason: https://en.wikipedia.org/wiki/Microsoft_POSIX_subsystem
Most cross-platform applications for desktops and servers. If you write to the POSIX standard, it will work on a wide range of systems. Many applications don't need to go beyond that.
Historically, the US Government. NIST mandated that only those operating systems should be used where existing applications can easily be ported to and fro, and POSIX compliant OS-s tick all the boxes. Windows NT got its POSIX subsystem for this reason: https://en.wikipedia.org/wiki/Microsoft_POSIX_subsystem
Which was hobbled to the point of uselessness are purely a box ticking exercise.
Most cross-platform applications for desktops and servers. If you write to the POSIX standard, it will work on a wide range of systems. Many applications don't need to go beyond that.
Anyone who either develops or requires POSIX (now UNIX) compliant services or tools?