Pipe clamps are very inexpensive compared to parallel bar clamps (also called K-body clamps), and have a virtually infinite range of clamping capacities because you can always replace your pipe with a longer or shorter one, or extend it with a pipe union and another pipe. With a parallel bar clamp, you're stuck with the length you originally purchased.
You can also use cauls with pipe clamps (or any type of clamp) to produce even pressure across a surface. One type of caul more or less extends the jaws of a pipe clamp, while curved (or cambered) cauls are commonly used to distribute the pressure from two clamps across a long edge or face.
