Unlike celebrity culture, popularity in the open source world translates to actual impact on the web. As an author of a popular library, your code plays a direct part in how other developers structure their codebase, and -- depending on the library -- the end user experience.

And, yeah, impact/change/popularity (whatever you want to call it) is certainly a main reason behind releasing and maintaining open source software. Perhaps other dominant reasons include giving users differently opionionated alternatives that better suit their workflow, advancing the technical know-how of a field, and simply experimenting for expressiveness' sake.