I'd say that the probable scenario here is that you're targeting some specific LTS Linux distros, and you want to be able to run on the specific version of glibc on those distros. Normally, binary compatibility goes forwards but not backwards. What this tool lets you to is target an old glibc without needing to install that specific glibc.

In my case, older versions of glibc (< 2.24) have a bug which prevents using posix_spawn. I'm not sure exactly which version of glibc has the fix. I would like to test against all glibc releases in turn, to identify the first in which posix_spawn is usable, but AFAICT there's no easy way to do that.

docker with older linux distros is a pretty decent way to do it