I can imagine it determining the rate of the whole reaction if it is the last step since that means that we will have lots of the reactants needed for the rate determining step, synthesized in earlier steps. Therefore, the rate of this step represents the rate of conversion of these reactants to our final products. But if it is in the beginning , you will have to include the speeds of the other steps in the overall rate which means the overall rate is not dependent on the rate determining step alone. So why is it called a rate determining step if it is in the beginning ?
NOTE : If it is the last step, then we only consider the speeds of the earlier steps until we have excess of the reactants which feed into the rate determining step. As for the rest of the time, we will only have a store of reactants which are waiting for the rate determining step.