It's almost always just a distraction -- a conversation with guests, thinking about how long till lunch, or just not feeling 100% that day and falling into "autopilot" mode. But a good driver will usually catch it before they're far enough off course for the guests to notice.
The computers don't give turn-by-turn directions like a nav system; they just have a text file listing the directions, available from a menu. The driver can't even look at it while the bus is in motion. Its really only useful at the start of a route you're not familiar with.
The really bad mistakes, like the PP's driver getting lost in the WWoS, or the legendary driver who got half way to Tampa before his guests convinced him to turn around, are very rare.
As for the OP's bus, those resorts are not usually combined on a route, so it was probably a very late bus in what I call "taxi mode." All the guests going to resorts that are close together are loaded onto a single bus, and it's up to the driver to work out the most efficient route. That driver just did a really bad job of it.