When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Yep. You need it.
That plate is there to direct the water flow efficiently. If the space between the impeller blades and the backside of the pump is too great, the water will simply swirl around inside, instead of flow through the pump.
Essentially, too much dead space doesn't allow the impellers to create directional suction and pressure to move the coolant.
So, you need that plate to ensure viable pumping.