Chad Callihan tempts Betteridge’s Law of Headlines:
When you’re specifying WITH CHECKSUM as you’re backing up databases, SQL Server will use checksums to help catch any inconsistencies with pages. This seems like a setting that you should always use and would expect to be a default setting. So why doesn’t SQL Server include it by default?
Using the principle that a backup isn’t valid until it’s verified, CHECKSUM acts as a useful but not sufficient check.