Jetbridges typically only load via the forward left door. Whereas you can attach stairs to the same door as well as the rear left door so embarkation and disembarkation is a lot quicker. Yes there are some choke points when folks board from the wrong door but generally, it's faster.
Pax of limited mobility are brought on via the forward right door (same as catering) and they use a similar scissor lift type of vehicle. (roll on/roll off).