Many reasons.
- Something that is of interest to me. May be I will use it sometime in future on my own project. For example, I have bookmarked wyswig editors (summernote) built using bootstrap. Might use them in future.
- Frameworks that I work with. E.g. Flask. Want to keep an eye on latest updates, issues, resolutions, patches etc.
- A developer/person that I want to follow. I star their repo that is most relevant to me.