The install script may not have all the context it needs to be installed. In the long run it is better to teach the user how your software works in plain english.

Even in projects with an install script, for example pmbootstrap, the install script also needs a tutorial.

In my experience, projects with minimal documentation and an install script will have the the install script fail halfway through because it assumed something about my system that isn't true, or it will do something incredibly insecure like requesting su and then curl | bash