 Why do Bob and Quezy haul asteroids in the first place?
Interplanetary Hauling Company. Not only that, we're rich! While Queazy—so called because his full name was Quentin Zuyler—dropped the ship straight down to the smooth surface of the asteroid, and clamped it tight with magnetic grapples, Bob flung open the lazarette, brought out two space-suits. And if The Interplanetary Hauling & Moving Co., hadn't been about to go on the rocks (chiefly due to the activities of Saylor & Saylor, a rival firm) neither Bob nor Queazy would have thought of sending an answering ethergram to Burnside stating that they would fill the order. And yet, if by some chance there was such a rigidly specified asteroid, their financial worries would be over. That they had actually discovered the asteroid, using their mass-detectors in a weight-elimination process, seemed like an incredible stroke of luck. For there are literally millions of asteroids in the asteroid belt, and they had been out in space only three weeks. The "asteroid in your back yard" idea had been Bob Parker's originally. Now that he and Queazy had found the asteroid, they were desperate to get it to its destination, for fear that the Saylor brothers might get wind of what was going on, and try to beat them out of their profits. Now they scuffed along the smooth-plane topside of the asteroid, the magnets in their shoes keeping them from stepping off into space. They came to the broad base of the asteroid-wedge, walked over the edge and "down" the twelve-foot thickness. He satisfied himself as to the exterior of the asteroid, and then sent the twin beams deep into its heart. A cold, completely disagreeable feminine voice said, "May I ask what you interlopers are doing on my asteroid?" He and Queazy caught up with her on the side of the asteroid they hadn't yet examined. "You want to move this asteroid from its orbit and haul it back to Earth. If we don't take this asteroid to Earth before June 2, we go back to Satterfield City and work the rest of our lives in the glass factories. Those boys are after this asteroid too, and they won't hesitate to pull any rough stuff. The big ship had landed, and little blue sparks crackled between the hull and the asteroid as the magnetic clamps took hold. A few seconds later, the airlocks swung down, and five men let themselves down to the asteroid's surface and stood surveying the three who faced them.
 It's a new fad that Bob hard started, where rich people enjoy having them on display.
 They are hoping to start a new business selling them.  Other companies have been making a profit with them, and they want in on it.  The asteroids of deposits of rich minerals, making them valuable. Hence why they check the composition of each one.  It's a new fad that Bob hard started, where rich people enjoy having them on display.