As with almost everything surrounding the topic, it's personal, if for you it's pointless, then don't do it.
Reasons for it may be, to explain why, how, goodbyes, expectations, hopes, wants, to confort those you left behind. It may seem pointless but to a lot of people it's the last essence they're leaving behind or probably the only chance of being extremely raw, vulnerable and having complete honesty, it's the only place where expressing feelings will have no consequences at all since the person that writes it will have passed.