My experience is that for internal documentation the time spent explaining things to a technical writer is bigger than the time spent writing the documentation
This isn’t the case for external documentation, that has to be more polished, needs sign offs and images and demos and stuff - tech writers can come in useful here
If I'm simply consuming a library, I find most real-world documentation to be pretty superfluous. A full working example is usually enough to understand how things fit together.
If I'm working on a library, design docs and commented code is nice though.
It may take more time to explain it to a tech writer than it would for you to write it, but in most cases the final docs written by a tech writer will be much better.
