I recently wound up rereading Jeremy Allison's A Tale of Two Standards (via Everything you never wanted to know about file locking), which tells an origin story for the POSIX standard for Unix where it was driven by ISVs wanting a common 'Unix' API that they could write their products to so they'd be portable across all the various Unix versions. It's quite likely that this origin story is accurate, and certainly the divergence in Unixes irritated ISVs (and everyone else) at the time. However, it is not the origin mythology for POSIX that I learned during my early years with Unix, so here is the version I learned.

During the mid and late 1980s, the US government had a procurement problem; it wanted to buy Unix systems, but perfectly sensible procurement rules made this rather hard. If it tried to simply issue a procurement request to buy from, say, Sun, companies like SGI and DEC and so on would naturally object and demand answers for how and why the government had decided that their systems wouldn't do. If the government expanded the procurement request to include other Unix vendors so they could also bid on it (saying 'any workstation with these hardware specifications' or the like), people like IBM or DEC would demand answers for why their non-Unix systems wouldn't do. And if the government said 'fine, we want Unix systems', it was faced with the problem of actually describing what Unix was in the procurement request (ideally in a form that was vendor neutral, since procurement rules frown on supposedly open requests that clearly favour one vendor or a small group of them).

This government procurement problem is not unique to Unix, and the usual solution to it is a standard. Once the government has a standard, either of its own devising or specified by someone else, it can simply issue a procurement request saying 'we need something conforming to standard X', and in theory everyone with a qualifying product can bid and people who don't have such a product have no grounds for complaint (or at least they have less grounds for complaint; they have to try to claim you picked the wrong standard or an unnecessary standard).

Hence, straightforwardly, POSIX, and also why Unix vendors cared about POSIX as much as they did at the time. It wasn't just to make the life of ISVs easier; it was also because the government was going to be specifying POSIX in procurement bids, and most of the Unix vendors didn't want to be left out. In the process, POSIX painstakingly nailed down a great deal of what the 'Unix' API is (not just at the C level but also for things like the shell and command environment), invented some genuinely useful things, and pushed towards creating and standardizing some new ideas (POSIX threading, for example, was mostly not standardizing existing practice).

PS: You might wonder why the government didn't just say 'must conform to the System V Interface Definition version N' in procurement requests. My understanding is that procurement rules frown on single-vendor standards, and that was what the SVID was; it was created by and for AT&T. Also, at the time requiring the SVID would have left out Sun and various other people that the government probably wanted to be able to buy Unixes from.

(See also the Wikipedia entry on the Unix wars, which has some useful chronology.)