Mainly temperature control, I think. Sperm production is optimum in a certain (and limited) temperature range. For this reason, the testes are pulled closer to the body in cold weather (most men will be familiar with the scrotal ‘shrink’ shock of getting into cold water – my brother called this the ‘’ooh Zone layer’) and the testes will be lower in hot weather, to get them away from the hot body core. Incidentally, this is why men who are trying for a baby are sometimes advised to wear underwear which is more loose fitting. I guess that scrotal inflation would mess with the bodies natural systems to regulate control, if only temporarily- but then all body modification is likely to do that.