1.) Redundancy. Can't get at my KeepassXC?, then I try Bitwarden. Can't get at my Bitwarden? Then I try KeepassXC. Can't get access to both? Then I'm screwed (but this is unlikely).
2.) Browser extension. I like to be able to login easily to sites using their extension.
3.) LOCKSS (Lots of copies keeps stuff safe). Having a cloud-based mirror of my login entries stored in Bitwarden is marginally better than solely relying on KeepassXC.
4.) Logging into KeepassXC is sometimes more complicated than logging into Bitwarden. I have a long seven word passphrase for KeepassXC, and use a keyfile which I have to 'hunt down' in the filesystem each time I go to access KeepassXC (I have disabled the ability to store the last used database & keyfile for security reasons). With Bitwarden I just open the app and if it's unlocked, I get quick access to everything.