A mother culture takes a 1/4 tsp or so of dry culture and grows it in warmed milk to a huge bacterial concentration. That populated milk is then frozen as cubes. When it comes time to make cheese, I take a bag of 8 cubes of a particular culture (in my case: Alp D, MA4001, Kazu, TA61, Aroma B) out of my freezer, thaw them in a bowl of lukewarm water (it doesn't take very long), and add them to the milk at ripening temperature (90F?). Result? Instant huge bacterial population, which leads to faster pH delta, quicker cheese process, etc.
If I had added that same 1/4 tsp of culture to the milk at ripening temperature (90F?), I would have to wait for the bacteria to wake up from their dried slumber and propagate to the same point that the cubes had when they were added to the milk.
Sailor made it all very clear to us how easy it is to do this when he posted his pictorial "how-to". It took me a while to see the light.