Regarding your second thought—one obvious way a checksum helps in gracefully handling invalid files is that a checksum pass/fail decisively rules out with a pro

Regarding your second thought—one obvious way a checksum helps in gracefully handling invalid files is that a checksum pass/fail decisively rules out with a problem occurred during the data transfer. For example, a checksum always you to know when it is/isn't appropriate to automatically attempt a redownload or tell the user to try to redownload a file.