Speaking as one who was first trained in this some 35 years ago there is a further aspect which hasn't been mentioned.
Nowadays with GPS and sophisticated autopilots, flying the approach is relatively straight-forwards, but in the days when it required rather more concentration it was all to easy for PF to forget about anticing, fuel states, QNH/QFE, crosswind, clearance to land, etc.
The monitored approach allowed the co-pilot to fly the A/C whilst the Captain did what he was paid for, i.e to take an overall view of the operation.
At decision height he either said "Go Around" in which case the co-pilot did that, whlilst he monitored, or "Land" in which case he was looking outside whilst the A/P landed the A/C.
At the time it worked well and allowed BA and others to get clearance for CAt3 autoland.
Whether it is still the "Gold Standard" is open to debate but at the time it worked well.