Optimum altidue increase due to reduce in weight and speed,as fuel burns off weight decrease and your 1.32 VMD(which ur minimum drag speed) also decrease so in order to get the best SFC(specific Fuel Consumption)u may climb to ur cruise climb which is ur Optimum altitude at this moment either once or using step climb from the FMS.The idea is to get the best speed for drag,consequently as ur fuel burns off while ur weight is deacreasing climbing to ur optimum altitude help u regain ur best Mach speed and make u save much more fuel than needed.