because that is where your oil is stored in the engine before it gets sucked into the pump & sent around the engine to do its job of lubricating & cooling of the engines oily bits, so to get the best idea wether your oil is upto the job or being pushed past its working limits you should ideally have the sender in the sump ,but it is harder to do i.e. welding a bung in the sump to take probe or do what tabetha said & tap your sump plug