The FP for shows are more time-specific for scheduled shows. For continuous-running shows, they give the normal time range. You go into a different entrance, and get to the waiting area. The standby line, which has been waiting outside building up, come into the waiting area (I call it the holding pen) too. You all wait in the holding pen together, until the doors open to let you into the show.
Oh, and in the holding pen, they have everyone "take 2 steps forward and 2 steps to the right" all the time. Therefore there are NO lines in the holding pen. When the doors open, the fun really starts, lol. Everyone shoves through the door (standby liners and FP liners alike). So basically, having the FP sends you through a different line to the holding pen, guaranteeing that you will get into the next showing.