Blender's Set Origin can only put the origin at the geometry's center, its center of mass, or the 3D cursor. To get an origin at the bottom of an object, you have to snap the cursor there first, one object at a time. QuickPivot puts the origin on any point of the bounding box (bottom, top, a corner, or anywhere in between) for every selected object at once. You see the result before you apply it. It also works on empties and hierarchies, which Set Origin can't handle.