I don't see how jQuery could have been anywhere near as ubiquitous if it had required a build step.
It makes experimenting easier for beginners, for people considering switching to it, for people who are already using it and want to try something outside the context of their existing codebase, for people writing tutorials and demos, etc.
The less bullshit to wade through when using a tool, the more situations it will be applicable to, regardless of skill level or project size.
