Distrobox changed the way I use Linux. I cannot imagine going back.
First, you are exactly right that it allows you to separate app repo from the rest of what you live about a distro.
I use an Arch Distrobox with every machine. Using Chimera Linux that uses MUSL, Clang, libc++, and BSD userland? Install anything from the Arch repos or AUR in seconds.
But it is not just package repo size. Using an app that targets RHEL? Install it from a RHEL Distrobox.
Doing dev for a project whose users are Ubuntu people? Build it in an Ubuntu Distrobox.
Want to try something and do not want it to mess up your system? Do it in a Distrobox.
Need some software for a class that will just be cluttering up your system after? Make a Distrobox for that class.
I have a .NET Distrobox. I have a Java Distrobox. Just not having to update the IDE and frameworks all the time is a huge win.
Mature application that I use every day that I do not want to change or break on me? Install from a Debian Distrobox.
Rapidly developing app where I want the latest for features and fixes? Install from an Arch Distrobox.
Tools you like that only Mint offers? Install a Mint Distrobox.
Distrobox is the greatest.