Sometimes it can be helpful to install additional Python dependencies into the Docker container before AppDaemon starts, to allow additional libraries to be used from Apps. The Docker script will recursively search the CONF directory for any files named requirements.txt and if it finds them, use them as input to pip3 to install any packages that they describe.
i think that Andrew has created a simular option in appdaemon, if thats what you mean. but thats in the dev version.
appdaemon doesnt have an UI (yet) that is coming in version 4. (which is in dev now)
the difference is:
appdaemon and HADashboard are programs written by @aimc
the addon is just a part from hassio that makes it possible to use appdaemon in hassio
and indeed you can set the packages that you want to pip there.
be carefull though, most packages depend on other packages and they give an error and then the addon breaks down and never starts appdaemon.
to see what the problem in that case is, look at the addon log.