i mostly use them for proprietary stuff or for software that is incredible painful to package (mostly electron apps). i will probably never use them for anything that actually matters but i also use rolling release distros everywhere so latest release is never too far. for testing latest version of any software i prefer appimages since they are simpler and don’t need a messy setup as flatpak, but i also won’t use them pass the testing phase and i prefer packaging the software if possible.
snaps, on the other hand, will never go near any of my systems. not even by accident
main selling points are isolation and having the latest version directly from developers without having to wait for your distro to package/update it.
both are debatable since they are not as good as promoted (isolation doesn’t always work correctly and it’s a mess to configure it once you use anything different than the more mainstream distros) or goes against the historical preference (using bundled everything instead of cooperating with your distro packages and trusting every individual over trusting your distro as a whole) but having the latest version on any distro without having to wait is a popular need so they gained traction quite fast. this might make little sense for rolling release distros (arch, nix) but it’s helpful if you have a stable base (years old debian) but need the latest feature on an specific application or have to use very specific libraries that are not packaged on the main distro and would require complex upgrades