It's so the project loads faster. Sounds which aren't played aren't downloaded at all, and music can often be very large and is streamed during the running of the game. I think this is important since often sound can make up the majority of the project file size. Having your project load in 1 minute instead of 4 minutes is worth a little latency on the first time the sounds are played, which you can cover up with preloading. And thanks to the offline cache there will be no delay on the second play.