This is probably one of the biggest mistakes a homeowner can make. For most cleaning, even when your pool looks like a toxic waste dump, you don't and SHOULD NOT drain the pool. When one drains the pool and there happens to be water under the shell (like in the rainy spring when people want to clean up the pool) the entire pool shell can heave. This is because the water under the pool creates an upward hydrostatic force (through buoyancy) and the pool is lifted out of the ground .
In some cases the only reason to partially drain a pool is to dilute the Total Dissolved Solids (TDS) or even stabilizer. These typically only require a partial draining.