The reason you have to give the QNH on initial contact with GMP/GMC is because you will be given a SID. That SID includes an altitude to fly, and when you are given an initial altitude you will also be given the QNH. That is why you are given and have to readback the QNH. Not because you heard the previous aircraft report "information Lima" and you have Bravo because you forgot to listen to the ATIS, so you lie and say you have Lima as well. Happens all the time.