The purpose of the bucket test is to compare normal evaporation to the actual leak in a pool. The idea is that the surface of the pool will evaporate the same like the surface in the bucket. If the water level in the bucket drop one quarter of an inch in 24h and the pool drops the same (1/4") then the pool does not leak. If you didn't do the bucket test you would think that the pool leaked 1/4 inch. See the video instructions below: