Setting up your requirements for heroku is not that simple. You have to create all the necessary information, like your requirements.txt etc. Turn on some level of real postgres, things of that nature.
Nicer framework generators will produce dependency configuration, have params for what database backend to produce configuration for, and produce applications prepared to be 12-factor (log to stdout, accept configuration via environment) out of the box.
More generators than those will allow you to create reusable app templates which have these features so you don't have to repeat the (still very simple) configuration process for each app.
Most of my time is spent consulting docs.