The reason being, documenting things like... time spent with children. "Disparaging" words. ie. potential slander. Financial concerns. In other words, "What you say might and can be used against you in a court of law".
It becomes a situation where they (or you, be careful) might say something like "If you do (insert bad thing here), I'll make sure you never see the kids again", because they might also be keeping a record of what you're saying to them such as "I'll make sure I drain the bank accounts before you see one red cent".
It is just a matter of protecting yourself, like you might if you feel there is a problem in the work place.
