Actually bookmarks describes my use of them pretty good. I browse /trending repositories at least once a day. I often find interesting projects in there that might come handy one day. But instead of bookmarking, I star them on github. I think that starred projects attract more contributors. Haven't checked on that though.
P.S. I usually DO NOT check the quality of that project prior.
I wonder if there are any interesting hacks around stars: like monitoring repositories for major releases, discovering similar projects.
And may be there could be some neat ones if we knew the social constructs associated with them.
To make the owners happy and more willing to continue working in the project.