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I think they re-enter that fast so that they aren't overly exposed to the heat of the atmosphere, but i'm probably wrong.
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It is the speed of re-entry that creates the friction that creates the heat.
Basically, in space, the orbiter, (or whatever), can, (and have too), travel very fast simply to orbit. Think about it, if you throw a stone vertically up in the air, it falls down again - gravity. Now throw the same stone horizontally, it falls to the ground, but some distance away. Throw it harder, (i.e. give it more velocity), it lands even further away. Now, consider this, if you threw the stone fast enough the stone would start falling to the ground, but the Earth is round, so it falls and falls, but the Earth's surface is "falling away" at the same rate, hence the stone never hits the ground - it has orbital velocity. (In practice you couldn't do this at ground level because the speed needed to acheive orbit would generate so much heat, the stone would vapourise!).
Next problem, the atmosphere. If you move something through the atmosphere, it pushes through the air. The air in front of the object has to rush around the object and take it's place behind the object, in doing so it creates friction, (more correctly called "drag" in aerodynamic terms), which creates heat. If you rub your hand accross a carpet for example, you feel your hand getting warm, rub it faster, you feel it get warmer.
Now, you have a space craft at orbital velocity in space where there is little or no drag, but you want it on the ground at zero speed. You have very little fuel to slow yourself down, because fuel is heavy, so what you do instead, is slow yourself a little, i.e. to just under orbital velocity, and you begin to fall in a long arc to the ground. As you get deeper into the atmosphere the drag, and the friction, increase, hence the the heat, but you also shed velocity since the friction slows you down, it acts a a brake, indeed, this is technically known as "aerobraking".
Now, as long as you can shield yourself from that heat, (which can be thousands of degrees), you get a free braking mechanism, a little fuel to keep the craft at the right angle, but otherwise, the atmosphere is doing all the work.
What may have happened to Columbia, is that some of the ceramic tiles which shield the ship from this heat have been damaged or knocked off, and the heat has got at the ship itself, damaging or melting parts of it. If that happens to a critical component, it can break, and cause a catastrophic failure of the entire system, remember, although it does not have orbital velocity any more, it is still going REALLY fast. (A Concorde airliner flies at Mach 2, these guys were doing Mach 18 when it started to go wrong). Loose a bit of wing or similar at those speeds and the thing will simply tumble out of control and break up in a fraction of a second.
I hope that makes sense. (To those who know more, yes, I know it is more complicated than that in practice, but that is essentially what is happening!).
