From what I understand the purpose of Fast Pass+ is to address the 2 big problems with the current Fast Pass
The way it is now you can book a trip a year in advance, make dinner reservations months in advance, but when you finally get to the park, your day is dictated by whatever time the Fast Pass machine randomly spits out. If the FP time conflicts with a parade, show, dinner reservation, fireworks, ect... you then have to change your plans to whatever this little machine tells you
The other problem people seem to have with it, is that you can only hold one at a time, but if they just adjusted the current system, you'd spend all of you time criss-crossing the park from this 1st kiosk to the 2nd one then back to the 1st ride, then the 2nd ride, and that's assuming all the randomly generated times sync up, which leads back to the first problem
So making ride reservations in advance lets you see the things you really want to see, you'll apparently be able to choose "morning", "afternoon", or "evening" so you can see it when it's most convenient for you, and you won't have to spend any time running around to kiosks
But that's the purpose of the new system, it's not to mess with the stand by lines, it to make Fast Pass more accommodating to people who spend a year or so planning a vacation to Disney World