I personally prefer to use Flatpaks over traditional packages because of the added security, sandboxing, and overall convenience of not having to deal with dependency hell. It's especially nice being able to have proprietary applications sandboxed from the rest of my system without worrying that Steam is snooping on my 'super-important-tax-documents'.

Flatpaks are also very useful for having up-to-date packages on distros like Debian, and it's derivatives. People can still use their preferred distro without having to worry about not getting a certain update, feature, bug fix, etc, for their applications.

Being able to restrict what applications have access to is a game-changer for me. A lot of times Flatpaks, by default, have very lenient permissions, and with the use of Flatseal I can restrict it to my liking. Worried about Audacity's telemetry?? Turn network permissions off. Now, not all applications will work well (or at all) without internet connectivity, but for applications like Audacity, it works great!! Flatpaks can also be very useful for developers.

That's not to say that Flatpaks are without their fair share of issues. Are they bloated?? Yeah, and although it's not an issue for me, it may be for some people. Desktop integration is, meh. Themes, and fonts don't always integrate the best. (A while back there were issues with Flatpak's sandbox, but I won't touch on that because I need to refresh my mind on it, and it was actively being developed to fix those issues so it possibly isn't even an issue anymore.)

Overall I think Flatpaks are absolutely wonderful.