It's also generally useless.

If you're building an open source general purpose tool, or something else meant to be reusable and consumed by the general public then sure. But the vast majority of software we write has a very definite lifecycle of birth, maintenance, and death. For the most part, one team with continuous word of mouth knowledge transfer will be responsible for it. And by the time that team has moved on, the software itself will have outlived its' usefulness. In an agile environment like this, keeping any kind of documentation up to date to be meaningfully useful is almost impossible without a dedicated team member.