The valve seats are set into the head surface sometimes upto 2mm and more. So the mixture coming out from under the valve will run straight into this wall created by sinking the valves in. Relieving is merely blending and radiusing this area to allow the mix to flow unheeded. You can get thinner head gaskets to regain any losses or shave the head. Mostly this port work is combined with a high comp piston and sometimes the compression actually needs lowering if you want to run pump fuel. Each job needs to be assessed as you build it.