As far as actual field use goes, they're principally long-range target pieces used by folks that have a fair amount of loose change to spend on ammo. A subset of this is their use as sniper rifles. Their disadvantage as sniper rifles is their size and weight, and that most of the ones popularly available are bolt-action and hence do not provide a rapid-fire capability. But for VERY long-distance engagement of either paper or person they're quite the thing.
They don't really shoot down airplanes, you know.
Tell that to the Luftwaffe.