They keep track of hours so that they can use this as a tool to push the rank and file to do more; more hours, more book studies, etc., because growth is necessary to maintain the cash flow of the WTBTS organization.
By keeping records, the organization has a quantifiable unit of measure with which to "crack the whip" on underperforming congregations. In my time, the CO would visit and was never happy with the performance of the congregation, in terms of hours of field service, books placed, etc. Guilt trips would promply be imposed by the CO on the entire congregation for not doing enough.
Although I was raised in this cult, I cannot understand why anyone would be stupid enough to believe anything they say. By the time I was in my early teens, I had already come to the understanding that theWTBTS is a cult. It always amazes me that there are those who are so blinded by faith that they can't see what is painfully obvious to anyone with a modicum of skepticism.