Unlike the FC cart situation in Japan where anyone could make their own carts, licensees had to send their completed games to Nintendo for duplication and mass production; the only disks the companies could use were these white ones as per the contract. The disks' price varied depending on how much royalties you were paying to Nintendo, but it's said that it was around 5000 yen (about $42) per disk. And licensees naturally needed a lot of these disks - not only for development purposes, but also to give to magazines for reviews and such.
Although all white developer disks have this same generic label, they sometimes come in cases or covers saying "Please return to Sun Corporation immediately" or other such statements that should be familiar by now to people who've been staring at scans of NES protos. And like regular protos, although the general public aren't supposed to have these quite a few disks have trickled into the hands of Japanese collectors.
Thx: Disk System 2nd; 2/99 issue of Game Lab