Virtually all piping systems go through changes in diameter either in the run or at the equipment connections. Since an expansion joint or flexible connector is needed anyway, it makes good sense to have the expansion joint serve the dual purpose of changing the piping diameter and handling the expansion, misalignment or vibration problem all at one time. Should you work with a straight connector, it means that in addition to the expansion joint, an expensive cast iron or steel reducer must be used for the piping transition. This would result in a need for additional space and add to the cost as well.