Essentially the byproducts are diluted out with replenisher. (The excess volume either overflows or is carried over into the following solution.) Compared to the processing tank solution, the replenisher is a bit overconcentrated with both developing agent and preservative. Also the pH is higher to boost the tank pH back up a bit.
In the systems that are replenished with original developer, obviously the initial tank solution must change during processing, and should eventually reach some different equilibrium levels. I'd guess that the B&W systems have a large error tolerance which allows this sort of thing. My experience has mainly been in large scale color neg and paper systems, which are generally too finicky for this sort of thing.
I guess if you were running 10,000 negs through a machine a month this would end up being economical. But I don't see why anyone would bother with it on a small scale. Just make a new batch.