You may not, but many people seek attention.

Good question. Some subset of the population is willing (wants?) to share such personal information (credit card transactions, etc).

Even then, users want to curate the info to present their best foot forward. Think of checking in to a nice restaurant instead of Taco Bell on Foursquare. Or sharing an iTunes purchase instead of an embarrassing one on Blippy.

While sharing their browser history, it seems rare that users would share 100% of their history uncurated. As long as they have the option to selectively show their history, users would be OK with it.

Asking you for what is already public would be no fun. People are forming various schemes to get you to part with your private data because that is what is valuable.

Something mysterious about being transparent I guess. I feel that there is a lot emphasis on "show the world everything and let the community reward you/scold you on your actions" type of mentality with these sort of services that entices some people.

...because they can make money by selling your privacy.