This is called "Wonging"...
Joining late in the game (shoe) is one method of advantage play used by Card Counters. Typically, one stands back and watches a shoe game and counts (known as Back-Counting), then enters play when the True Count is about some preset number, i.e., where they have an Advantage.
True Wonger's also leave the table once the run of good cards is over, i.e., the Count drops below some level.
Yes, the Count is always Zero at the start of each deck/shoe, but the cards do not know that. In order to get a +TC, you must play through (or watch the play of) Negative (i.e., Neg-VALUE) cards. Therefore, Wonger's increase their overall advantage by ONLY Playing When They Have the ADVANTAGE.
Wonging has disadvantages, depending on your goals for playing. For example, you may sometimes "count for hours" just to play a few hands. You will have to move alot, and I mean ALOT, therefore not a very good ploy if you just have one local store. You will not often get in enough play time for Comps. If you choose not to move frequently, you will be shown the door frequently, as it is pretty easy to spot a back-counter. Not to mention, THE PLOPPIES WILL HATE YOU, since you will "be spoiling the flow of the cards"!
Hope this helps answer your post.