GIGN uses these revolvers (often with both scopes and even bipods fitted as in this pic) for special purpose close-range sniping type situations, like trying to shoot a guy with a hostage from a building across the street. the pistol/revolver bullets have less chance to over-penetrate, and the mechanical simplicity of a revolver combined with the extensive hand-fitting and bespoke manufacturing processes of this specific revolver ensures it is reliably 'match grade' accurate at the needed ranges (each comes with a 25m factory test target) with little chance for jams or malfunctions.

also the 'jumpsuit + balaclava + unarmored tactical vest' look will never stop being the peak tactical aesthetic.

Here's an interview with the GIGN founder with some discussion about the MR 73

For the revolvers in general, it has to do with reliability and usage of specific ammunition to avoid overpenetration. While today we generally consider self-loading pistols to be sufficiently reliable, this wasn't necessarily the case in the 60s and 70s, especially with the aforementioned types of ammo. Additionally, pistols with large-capacity magazines were just starting to become a thing, so a 6-shot revolver didn't actually have that much less ammo compared to most other pistols of the time.

For the scoped version specifically, it had to do with sniping in urban conditions, where you don't necessarily need a very long effective range, but you do want your weapon to not have a long rifle barrel sticking out of a window, giving away your position.

Here's a video by Forgotten Weapons that goes into more detail: https://www.youtube.com/watch?v=P1zEUGck8NE