During the 747 development, they were so concerned that a pilot could not taxi an airplane this big, from such a high viewing perspective, that they built this test rig. Of course they discovered that it wasn't all that hard to taxi the 747.
Also, since it had 4 engines, Boeing would use the 747 to test new engines, like the really big ones for the 787. So the 747 would fly around with one really big engine and 3 of the regular smaller engines.
