It’s essentially a guarantee that certain tools and system APIs will be available, accept certain arguments, and accept/return data formatted in a specific way. It’s important for general purpose software and portability. Basically, if you want your software to run on anything and everything, you target POSIX. Except Windows, obviously, but pretty much everything else is POSIX compatible, or close enough that you’re probably fine.
An example: I write medical applications that are distributed to many hospitals, so well outside of my direct control. One program in particular is run on Windows, Linux, or AIX. I have to write separate handling for Windows obviously, but all I need to do to support both Linux and AIX is stick to POSIX APIs and test on Linux. Which is great, because I don’t have access to an AIX system. I still have to do final testing on AIX, but with one minor exception in the last five years, it all just works.
It’s essentially a guarantee that certain tools and system APIs will be available, accept certain arguments, and accept/return data formatted in a specific way. It’s important for general purpose software and portability. Basically, if you want your software to run on anything and everything, you target POSIX. Except Windows, obviously, but pretty much everything else is POSIX compatible, or close enough that you’re probably fine.
An example: I write medical applications that are distributed to many hospitals, so well outside of my direct control. One program in particular is run on Windows, Linux, or AIX. I have to write separate handling for Windows obviously, but all I need to do to support both Linux and AIX is stick to POSIX APIs and test on Linux. Which is great, because I don’t have access to an AIX system. I still have to do final testing on AIX, but with one minor exception in the last five years, it all just works.