Good documentation is a highly-scalable way to transfer knowledge. Writing it takes more time and effort than explaining things to a single person, but once written it can be read by any number of people. So it’s worth writing if you expect at least a few people to benefit from it.
My rule of thumb is, “If you explain it twice, write it down.” If the same question shows up from at least two colleagues, make sure the answer is available on the internal wiki. If two different customers run into the same issue, make sure the solution is in the public KB.
Once the information is written down and findable, the next customer or colleague who runs into it won’t have to ask - and when someone does ask, you can still save time for both of you by sending the already-written explanation.
I’ve heard of Product Support organizations that mandated a KB article be written from every support ticket, which surely resulted in useful articles but also ones that were never referenced again. Not every question or issue that comes up is likely to recur and it’s a waste of time to document one-time flukes. But most things that come up twice keep coming up.