You are right that they are designed to prevent spoofing. The problem is that the recipient server needs to be configured to use it and not all servers do, for

You are right that they are designed to prevent spoofing. The problem is that the recipient server needs to be configured to use it and not all servers do, for reasons spanning false positives to ignorance. So if you have a fairly old or known domain you will get non trivial amounts of backscatter from spoofed email being sent to invalid or otherwise rejected addresses.