So you can pour the concrete without permanently fixing the pipe by bonding to that concrete. The pipe and concrete will expand and move at different rates, which if bonded together, one might damage the other, or initiate cracks in the floor or piping. You won't usually wind up with a liquid tight seal either. AFter the concrete is placed, fill the box with a silicone, etc. type mastic, if desired, to maintain a flexible seal, where the pipe can be pulled and repaired easily at a later date.
