This is old discussion, but I will comment anyway. This phenomenon is real. Drives store bits using magnetism. Over time the strength of the magnetic signal weakens. Eventually it will weaken to the point where it cannot be read by the drive head. When you're using your hard drive and it's reading data back it is also measuring the strength of the magnetic signature. If it finds that the signature is weakening it will re-write the data back to restrengthen the magnetic signature. If this happens once in a while, no big deal. As your data ages on your hard drive this can be happening all the time. Now imagine the performance impact of your drive not only reading data, but rewriting nearly everything it's reading. It slows things down. This is often why people say a format and reinstall improves their computer performance, because they worked around this issue on their own without realizing it.
Interestingly enough, there is a similar circumstance with SSDs. They store electricity in cells instead of using magnetism. The cells hold charges with an insulator. Over time, the electrical charge "leaks" out of the cell and the SSD, detecting the issue, has to rewrite the data to restore the charge. This is also how SSDs wear out. Each time you force a charge through the insulator, the insulator is weakened a little bit more. Eventually it can't hold enough of the charge for the drive to read back out and that cell is then "bad".