alt Hacker NewsIt was about making software easier to port. You still couldn't necessarily just write software for all operating systems at once, but you had less porting work because less stuff was different because of the standardisation. So open(argv[1], O_RDONLY) works on all POSIX systems, but if you want to create a pseudoterminal, it's different on each, and if you want to create a container, they don't even use the same concepts.
> but if you want to create a pseudoterminal, it's different on each
This is somewhat outdated information. POSIX.1-2001 added "posix_openpt" to create a pseudoterminal, and most POSIX implementations now support it–at least Linux, macOS, FreeBSD, NetBSD, OpenBSD, Solaris, z/OS, AIX, HP-UX, QNX, Minix and Cygwin do. (Of course, that's only true of current versions, if you go back a decade or more you'll find many of them hadn't implemented it yet.)