GM-purpose

User scripts, ultimately, serve whatever purposes scriptwrights and user script
users have.

On a higher level, though, they evolve and democratize the web faster and sooner
-- by letting anyone and everyone partake in extending, improving, specializing,
generalizing and/or otherwise fitting it better to their needs and wants, by
themselves, without (at least technically) ever needing to ask anyone's
permission, or having any privileged access to the servers that deliver web
content and functionality.

We like that, and consider it a good intrinsic property of the modern web,
similar to how the ease of access to and copy-and-paste-friendliness of HTML and
javascript code that made the web such a powerful, fast growing and pervasive
medium in the first place. User scripting is a present day manifestation of the
same idea.

Creativity and usefulness over politics.