The idea in replenishment is that the optimal tank solution has a specific amount of active developing agent, specific pH and specific amount of halides. When you process paper, it releases halides from the paper and eats up some developing agent. "Replenisher" is a special developer solution that has little or no halides and more developing agent than tank solution. Then, as you add this replenisher into used tank solution, it stays at correct levels. But again, when you make a new tank, you can't use just replenisher but you need a tank solution. To make it easy, tank solution can be obtained by mixing replenisher and "starter". This information applies to all "replenished" processes.
So, simply put, if you use just replenisher as a new tank solution without a starter, you get too active developer with too little restrainer. This, again, is general information that applies to every replenished process.
PE has suggested that you don't need to use any starter with these RA-4 chemicals when you process at room temperature. You get great results that way. This is specific to this process and at room temp. I don't know if you need to use starter at higher temperatures. Maybe, maybe not. Probably you will get higher contrast or shorter processing time if you omit the starter.