Final answer:
Slight leakage from the stuffing box is required to lubricate and cool the packing, which prevents seal failure and overheating.
Step-by-step explanation:
The reason why there must be slight leakage from the stuffing box is to lubricate the packing and to cool the packing. The stuffing box is an assembly used in various pumps and valves to contain fluid within a device by wrapping around the shaft that passes through it. A controlled amount of leakage is necessary to ensure that the packing material remains lubricated, thus preventing it from drying out and cracking, which could lead to a failure in the seal. This leakage also helps to dissipate heat generated from friction between the moving shaft and the packing material, maintaining a temperature that prevents both the shaft and the packing from overheating.