Just like with GPS, you enter destination and go. This feature is missing in OneBusAway.
I am a daily user of one bus away, always trying to find alternate routes to avoid waiting for a long time. Also, newbees don't know their routes, only know where they want to go.
Just like stops, destinations could be saved in a list and by chosing a destination, trip options are offered based on arrival time.
The classic trip finder is not very usefull on smartphones, where you need quick trip info from where you are now to anywhere you click on map, at that exact time.