Those of us without luggage in the hold and travelling onwards from the airport by public transport know from bitter experience that buses/trains tend to leave maybe every 30 mins or every hour; miss that onward connection and you can have a rather long and annoying wait
Even if there is a need to board a bus from aircraft steps to the terminal it is still worth being quick to deboard the plane - at many airports if there are 2 buses to ferry pax, the 1st bus will often leave once full and not wait for the stragglers; boarding the 2nd bus can mean that (allowing for the queue of 90 pax to be processed by immigration) it takes an extra 10 mins before reaching the bus stop / train station
If going somewhere as part of a daytrip or two day weekend, a delay of half an hour in the middle of the day really does eat quite significantly into the time available when shops/businesses/museums are open