A long time ago I looked it up for it bothered me. JWs reason that the first century Christians kept records. For example, three thousand were baptized (Acts 2:41). At that stage, the answer satisfied me because I trusted them and did not look for ulterior motives or hidden agendas.
Now I see it in a different light. Firstly, if you look carefully in Acts, you will notice they kept tally, but not accurately. Acts 2:41 NW says "about three thousand priests were baptized." So that was an estimate. Secondly, it concerns those who were baptized, and had nothing to do with reporting of time.
Conclusion: As was spelt out above, JWs use it as a gauge to measure spirituality. Who is spiritually weak? Avoid those. Who is spiritually strong? Appoint those, etc. It is all about control.