Use cases for using old versions of packages in container

[znavko]

Hello! Will you bring some links to the materials or projects or fosdem videos 
where I can clear why is to use old versions of packages? Why developers need 
this feature?
Sorry, I had not found such messages in Russian linux forum about how to setup 
environment of previous software versions. The only discussions I heard were 
how to update without breaking the system and which package manager breaks less 
often.

Even the Reproducible Builds Project [1], connected to one of GNU Guix 
features, does declare the purpose like verification that code has no backdoors:
"The motivation behind the Reproducible Builds project is therefore to allow 
verification that no vulnerabilities or backdoors have been introduced during 
this compilation process."

I only want to find areas and to read about it, where OS users need to use old 
versions. I suppose it is embedded systems. But may be something else?
[1] https://reproducible-builds.org/