J.F. Rutherford began this tradition. In the beginning, all approved for field service were required to turn in a service activity report weekly. Of course, back then, all baptized Bible Students (and later JW's) really, truly believed that the end was upon them. They went out in service ALL THE TIME - no leisure time at all.
The congregations were called "companies" and pioneers were called "colporteurs" - which literally means book and magazine salesman. So the members of a local company were turning in their weekly sales reports.
Time slips later became a way to assess a persons "spirituality" - which is its primary purpose today.
ginger