Whilst unlikely there is always the chance that one of the pilots being checked is so awful that line trainer doing the check wants them out of the seat and continues the flight as an operating pilot.
An LTC would be qualified to operate from either seat, but an F/O couldn’t take over the captains role, which could cause delays. That’s apart from obvious CRM issues with an assessor who isn’t qualified to do the job of the person they are assessing trying to give guidance or training in the debrief.