2 sets of steps used for pax boarding/disembarking for a 737 800 or a 757 for instance is much quicker than an air-bridge/jetty as long as the ground staff get both the front and rear steps on PDQ...
Also today the ramp ground staff also have to faff about for ages (for H&S reasons gone mad) in setting up an elaborate chain link barrier system and cones to stop pax walking under the wing - this also wastes time on a quick turnaround if the ramp guys don't arrive with the kit or it is not left on the stand (more junk on the apron)
Back in the day the ground staff girl/boy boarding/meeting the flight, (or the No.4 CC would get off first) and would stand at the end of the wing to guide the pax accordingly - it's not rocket science lol