If you're asking this question, you're probably french, or lazy, or new, or something else :).
The initial time hit for writing a page of documentation is minimal compared to the time it saves others. A quick paragraph or two explaining why something is the way it is will allow someone else to quickly obtain information. If we need to know where a customer's systems are located or how to access the network statistics, then we don't want to spend a few hours trying to figure it out. That's time better spent doing something fun or productive instead of duplicating what the last poor schmuck went through.
There is (unfortunately) a ?school of thought that believes if you want to know something, you should duplicate the long solitary learning process. This is how information is lost. This is why there are fifty ways to do something that are shared among a dozen people with sole knowledge of them. This is the path of fail.