Normally, patch creates a backup file if the patch does not exactly match the original input file, because in that case the original data might not be recovered if you undo the patch with ‘patch -R’ (see Reversed Patches). However, when conforming to POSIX, patch does not create backup files by default. See patch and POSIX.
The --backup (-b) option causes patch to make a backup file regardless of whether the patch matches the original input. The --backup-if-mismatch option causes patch to create backup files for mismatches files; this is the default when not conforming to POSIX. The --no-backup-if-mismatch option causes patch to not create backup files, even for mismatched patches; this is the default when conforming to POSIX.
When backing up a file that does not exist, an empty, unreadable backup file is created as a placeholder to represent the nonexistent file.