There's nothing wrong with installing a .deb manually.

Personally, I'd hesitate to add any third-party repos unless there is a very good reason. In this case, the only real difference is that you won't get the updates automatically with sudo apt update; sudo apt upgrade without the repo. Either way, the desktop app will notify you when updates are available. There's very little advantage to using the repo.

Adding a repo is very rarely required. It has deeper consequences than simply installing an app, and requires a higher level of trust. If you don't understand the security implications of adding a repo (and its associated key), then my advice is: just don't.