The purpose of time slips is to measure one's level of dedication to the cult. It has nothing to do with charitable status (at least in the US) as is pointed out. A recognized religion has automatic tax exemption in the US. No charitable work is required for them since providing religious instruction is one of the criteria.
The measurement of time is the SINGLE most important criteria to gaining additional "privileges" for JW males. That's the first thing the elders look at in appointing men and you can exhibit every other "quality" needed as a MS/Elder but if you don't have 10 hours a month and usually show RVs and good placement history, then it doesn't matter. It's an easy way to measure "spirituality".