Preparing starter for use

I've got a question about the Tartine basic recipe. In the book, they call for feeding the starter regularly for maintenance by discard 80% of the active starter, then feeding with 100% hydration. Then, when you're about to mix a batch of dough, they say to feed the starter using only a tablespoon of the active starter. What is the purpose of the decrease in amount of active starter immediately prior to use? Is this preparing the starter for the 'shock' of a sudden increase in sugars?