What to include in a backup? (Ubuntu)

Hi! A friend just recommended the backup tool that comes with Ubuntu. I took a look at it and was wondering what you guys include and exclude from the backups. I just installed wire guard VPN and but the config file in the etc/wireguard folder, where it belongs. I would have to include this folder as well if I want to keep my configs. And I guess many programs do the same, so how do you know what to include, so you can just revert to the last backup if something breaks or you get a new machine? Maybe that is a stupid question, but it was going through my head for some time now. Thanks a lot!