The delay clip is a special part designed for the sector gear, whose purpose is to lengthen the contact of the tappet plate with the sector gear. Thanks to the longer contact the nozzle of the gearbox stays away from the hop-up chamber longer, which means that the magazine has more time to feed the BBs into the chamber.
It is recommended that the part is used with especially fast motors and gearbox build. Note also, that a highly efficient battery might increase the Rate of Fire to the point where using a delay clip is recommended. In that case, you can maybe avoid any blank firings and the gun can shoot more continuous and faster ROF.