I was probably one of the worst type 1's ever to keep a bg diary, thank havens for the modern bg meters where you can download your readings.
As Nyxks says, it's handy to record all your results as it helps your HCP's advise you better on your insulin doses, also if your confident enough to adjust your own insulin then you can do so yourself based on the information that you gather. There's some really good bg meters around at the moment that assist in this (Bolus Advisor's) one such meter is the Accu-Chek Aviva Expert, it can advice on bolus doses given the information that you put in, and can also advise on correction doses and such, a really good meter and you should ask your DSN if their clinic has them.