What is the difference between export and backup?

I am using DeltaChat for some time with my family but never managed to get the difference between “Manage keys” and “Backup”. I get that via “Manage keys” I can export my private key but what does backup do on top of that? When I dump a backup the resulting file is ~1MB whilst a exported key is just a few KB.

I guess, a backup contains all chats data.

Ok makes sense but I thought since all mail is on my server I would not need them to be included in a backup.