Well, I understand your reaction, but it may help to point our that there are three different algorithms involved here and that to ensure content completeness and ensure security concerns (CIA, NR, etc) you want them all, and all done correctly:

  *  Checksums, like in Xmodem or CRC
  *  Cryptographic hashes (including MACs)
  *  Cryptographic signatures (ie OpenPGP key or cert)

As noted in the other comments these protect against different kinds of problems in transmission, but in used correctly in combination can protect from both glitches and active attacks.

To say that it is difficult to implement all of these correctly and in concert is a grave understatement, but this is what modern crypto software and network protocols that use it, have to do.

Now back to the thread on HTTP header checksums :)