It's so that you can space the time you play on it.
The problem was that you had to have it running to gain XP. However, there's not enough to do in the game to warrant you sitting there eating up browser resources for hours to gain XP.
Now, you book the transport, and come back when they're due to arrive. You get many hour's worth of XP and Gold in one five minute hit, rather than sitting with it open for hours.
It also stop people *coughjerrycough* from leaving a browser open for hours on end just raking in the XP while they go to bed and stuff