That is an outdated part of code which is not even used in magento 2.
It resulted from the fork of the composer installer for magento 1 which has multiple additional features to extend magento 1 via composer.
This specific feature allowed to put php libraries installed via composer into a director, analog to \lib\ and also added a static autoloading for them, allowing deployment or even comitting them to the vcs without the need for others to also use composer.
This feature was/is experimental und was used from nearly noone
this answer concentrated on deployLibraries setting, the deploy manager itself was just a logical processor for all files copied from composer to the magento directory, so it was able to do this process outside of the old installer but as an plugin event