Think of it as multiple user lands on one distro. So, why?

What if I really love the stability and familiarity of Debian Stable but applications that are either ancient versions or totally missing in Debian? Well, I could use Distrobox on Debian to get access to the Arch Linux repositories ( the largest collection of Linux software anywhere ).

What if I do not want to install huge Flatpaks for the above and prefer more traditional package management and more timely updates?

What if I want to setup a dev or test environment, want to be able to make a mess and remove it later, not also want access to my other tools and files from that environment?

What if i want to experience other package managers and userlands without having to lose my existing system only to discover I do not like it?

What if I want to build software for a specific disto and want to access files and tools on my main distribution to do so?

I am sure there are many, many more reasons.

Anyway, it is just one way to do things with pros and cons like anything else. Personally, I really like it.