This feature is akin to using a shift function in a tilt shift lens (or sometimes known perspective control). Typically, if the camera is levelled parallel to ground, you wouldn't have "keystone distortion", meaning the verticals remain straight. If you tilt upwards, building will seem to converge, tilt downwards, buildings will diverge.
Here's a shot at 12mm on the 12-40mm /2.8 lens. As can see on far left, camera is placed levelled parallel to ground with help of level guides in camera, at the position I stand, I can't have the top of the tower if I want to have the building verticals remain straight. So the only way to have the top of tower is to tilt the camera upwards to capture that, however the verticals now is slanted and converging, on the far right is the Raw shot, which retains the original file. The middle shot is after keystone compensation was used to correct it. So as can see, with this feature it's possible to have both verticals straight up, and capture the top of tower. Hence you can achieve this right off camera (though jpeg only), without the need of TS lens/post process.
Hope the explanation is clear enough.