It just means that you add 300ml of replenisher for every qty 30 8x10 prints (or equivalent), and then at the end of the printing session, top off the storage container (as needed, it should say).
It does introduce some variability, but keeps the developer from getting oxidized. If you use less than 30 8x10 sheets in a printing session, just top it up at the end. I'm not sure it's hugely critical. It has worked really well for me whether I make one print or ten prints. Activity is really stable.
If you print very rarely, why not just mix LPD gallon kits with boiled distilled water and keep the stock solution? Store it in 500ml amber glass jars that are completely full, and just use one up each time you print. If you must, after you dilute it 1:1, store the working solution between printing sessions too for even greater economy. But to me it's penny wise and nickel stupid, because the expensive part is the paper...
The rationale for me is that I love the replenished developer. It seems that the left over bromides help the prints look nicer and more according to my tastes. It's a bit softer than straight 1:2 developer, the highlights are a little softer, and the shadows seem to carry a bit more detail. I can't quite put my finger on it, but it's what I'm used to since a few years now, and am happy to just continue with it, since my negs are tuned to my whole process now.