They may be used to install a specific older version of some software.
The test server is used to make changes and install new versions before "going to live."
For instance, in certain places, they decided to install differently mixed versions of the same musical passage.
A plan was devised to install a version of the work, about twice the size of the original, on an appropriate site in the city.
I recently installed a new version of a popular facsimile program, and it works worse than the old version.
Through this model, there should not, in theory, ever be a need to overwrite an aptosid system by installing an entirely new version.
In the 1980s many telcos installed an early version of these instead of pulling more cable into a remote area.
This is usually done at the beamline, but users can install a version of the Python tool on their home computer for specialized use.
Walsh installed a modified version of this system when he became head coach of the San Francisco 49ers.
I typically prefer to use virtualization because it makes it easy to install multiple versions at once.