There is a wide range of types of design docs, and none of them are useful. I've rarely seen any useful design doc at Google. I feel that design docs are for engineers who are too much process-oriented.
Here are the types of design docs I've encountered in the wild:
* The promo design doc: it's not really explaining what this is trying to solve, it's more stating that this project is awesome, and makes the company better. The logical conclusion is that the author of this doc should be promoted.
* The turbo-encabulator[1] design doc: this is a technobabble design doc which is full of terms never encountered before, and which is not understandable unless you're a senior member of the team. I'm sometimes not even sure the senior members of the team understand it...
* The new-grad design doc: this a design doc with no substance, but as the person just graduated from university, they felt compelled to make is as long as possible, to prove... I don't know what... It is not conveying any information. They most likely copied/pasted huge chunks of the code they've already written, to fill most of the ~70 pages of the doc.
* The made-up-facts design doc: this a design doc full of "everybody knows that", "they all say". Of course, it's not as obviously done as some politicians do it. But the design doc will push their design with "this follows good practices", "this software is slow, therefore..." who defined the good practice? why is it a good practice? what is slow? was it measured? is it an end user feeling?
This is 99% of the design docs I've seen out there. Of course, exceptions exists, but they're very rare, in my experience. I'm shocked that the author pushes for this practice... But again, they were no engineer, they were a director, I guess design docs make sense for their position, for which I'm still trying to figure out the value these folks bring.