The advantage of having a diary is that you can refresh your recollection of the events after an accident
While some lawyers may suggest you keep a pain diary on a day-to-day basis to record the problems you are having after an accident, the decision to have such a diary is really a personal decision. The difficulty with having such a diary is it focuses you on your problems, which is not a good idea from a psychological standpoint. The advantage of having a diary is that you can refresh your recollection of the events after an accident when you are asked about them years later.
If you choose to use a diary, what you should do, as a minimum, is keep at least a calendar to show when you attended various appointments. The calendar should also note significant events that occurred after the accident such as when your back problems stopped bothering you, when you returned to work, when you saw a specialist, etc.…
Note that if the diary is not kept for the main purpose of litigation and/or on the request of your lawyer, ICBC has a right to obtain a copy of the diary. Once ICBC gets the diary there may be plenty of evidence in it to be used out of context against you in the injury claim such as notes of improvement or recovery.