Indeed. It's true that the data may be corrupted before hitting any disk[1], but once it has hit the disks (>1), it's extremely unlikely that you'll ever hit a similar bit error where it'll mistakenly choose the wrong disk block to recover from.
The main point of e.g. ZFS or Btrfs checksumming is that a) at least it isn't getting worse, and b) I can tell if it's getting worse.
[1] ... but if the bits are not generated by the machine that actually saving to disk, how do you know they weren't corrupted along the way? The number of people who religiously check PGP signatures/SHA256sums or whatever is miniscule.
The main point of e.g. ZFS or Btrfs checksumming is that a) at least it isn't getting worse, and b) I can tell if it's getting worse.
[1] ... but if the bits are not generated by the machine that actually saving to disk, how do you know they weren't corrupted along the way? The number of people who religiously check PGP signatures/SHA256sums or whatever is miniscule.