Why do the match runners bother with shotgun?
Because otherwise it would be called two gun.
I mean, nobody is going to actually carry both rifle and shotgun. The shotgun can't do the job of either the rifle or the pistol. By that, I mean that it can't really handle cover users beyond 50 yds (25 yds without a choke) or dodging, prone going men much further than 100 yds.
Almost nobody is really fast and accurate with 12 ga slugs. Slugs cost too much to practice enough for that to be the case. If you were going to go to all that expense to fire a single projectile, why not just use the rifle? Also, when you actually check out the hits on a silohuette, you will be aghast at how little effective range a riot 12 ga has. Ditto how small that pattern is at 5 yds and less. So the buckshot spread is very little help at hitting anybody (at typical defensive ranges). Of course, what's true of a riot gun at 5 yds is also true at 10 yds, if you have a really tight choke.
Likewise, the shotgun can't be swiftly drawn from ccw, or handily carried that way, nor is it worth a hoot when fired with just one arm. Nor is it practical for use in really tight quarters (ie, Vietnam tunnel sort of deal) or while decending or ascending cliffs, trees,ladders, and so on. Nor is it handy while you are in your sleeping bag.
There are 3 basic types of modern firearms.
Handguns, shotguns, and rifles.
It is a competition to showcase ones proficiency in all 3 disciplines.
It is not meant as a what to carry in case of a doomsday scenario or extreme hunting/ self defense challenge.