The usual advice in mounting a scope or sights is to get as close to the barrel as possible. For a scope, that means selecting a mount that just allies the bell of the scope to clear the receiver with maybe enough space to allow a scope cover to slide over. The reason is simple: the farther apart the axes of the scope and the barrel, the greater the error in aiming if the rifle isn't held perfectly level.
But the are some cases in which there's an advantage to a high mount. One is with air rifles, like the Air Arms Pro Sport seen above. Pellets travel a lot slower than bullets, and have a lot more drag. Consequently, their trajectories are not as flat over their useful range as would be a .22 or a small caliber, high velocity rifle bullet. A 12-16 foot-pound gun like this Pro Sport, if zeroed at 22 yards, is going to see a drop of close to two inches between that distance and the maximum useful range of 50 yards. That's a lot of correction to dial into a scope- 16 clicks in a quarter minute of angle adjustable scope.
But if the scope is mounted two inches above the barrel, it's alignment is going to be a lot closer to the downward arc of the pellet than if it was mounted lower. A lot of field target shooters and some airgun hunters mount their scopes high for this reason. A second advantage is that on some stocks it's easier to get a good sight picture with a high mount.
There are some disadvantages. The earlier mentioned problem of error increasing as the gun is tilted is one. Another is that some scopes don't have enough adjustment range to deal with a high mount; in those case scope rings can be shimmed, or you can purchase an intermount from Barska and other suppliers that has a built-in angle correction. Such mounts are designed to correct for the "barrel droop" seen in some break barrel guns, but they work well in this application, too.
High scope mounting isn't appropriate for all applications, but it might solve a scope problem you have. Don't be afraid to try it just because the experts say you should always mount a scope as low as possible.