[Deregibus]

The checksum is only telling you when the value is probably incorrect, it doesn't show that it's correct or meaningful.

For example (on a completely hypothetical filesystem/disk), lets say you want to open a file. You read the entry in the filesystem metadata that tells you on what sector that file begins and it gives you a value of 5005. Even if the checksum is correct, the filesystem code still needs to check that a) 5005 is actually within the range of valid sectors, b) that it actually makes sense as a sector value (maybe files must always start on a multiple of 4), c) that the data at that sector actually looks like a file, d) etc.

If you don't update the checksum as well when generating the test data then you limit the amount of depth in the testing since most errors would get caught immediately at the checksum validation stage.