If we start from the understanding that the first officer on a commercial passenger jet holds the same licence and is equally well qualified to fly the aircraft as the captain
This is not necessarily true. There is an experience requirement to obtain an "ATPL" which you need to command a commercial jet. These days new F/O's join airlines with a "frozen" ATPL which when they have sufficient experience can be "unfrozen" by the authorities.
In the past, it was only necessary to hold a CPL to fly as an F/O on a large aircraft and you needed to take additional exams (and have the relevant experience) to upgrade your CPL to an ATPL. Some F/O's are still flying with only a CPL having had "grandfather rights" granted to them when the rules changed.
I understand that insurance companies offer reduced premiums if airlines dictate that some of the more challenging approaches are flown by the Captain (whether that's due to the airport or due to meteorological conditions - strong crosswinds etc - prevailing at the time).