As an example, RAR provides parity records which allows recovery of large amounts of compressed data in case of data corruption. 7zip can lose all its content if one bit is flipped.
RAR provides much more support for underlying file system support which makes it more suitable as an archival tool. Things like NTFS hard links, streams, ACLs, all three timestamps. 7zip doesn’t support that.
WinRAR in general has way more niche features for advanced use cases, while 7zip focuses on the basics.