Very handy indeed @Jeff Backes
! Don't lose those
If someone 'accidentally' presses that brake lever on a hydraulic brake after you remove a quick release wheel, the pads freeze together. That requires a minor brake bleed to pull the pads apart/relieve pressure on the pistons , something that's handy to know how to do (constant exposure to hot temperatures can also freeze those brakes) but in many instances means handling a fairly toxic brake fluid. If you don't have the ready made plastic separators, a fairly thin wooden shim or folded over piece of thick paperboard make decent substitutes in a pinch.
Transporting a bike with hydraulic brakes without a spacer is a risk, since it's all to easy to have one of those brake levers actuated while loading or unloading a bike or piling more bikes into a vehicle or driving over a bumpy road.