You rely on others to use and store your data responsibly.
Sometimes you don't know what your data is being used for and sometimes it's not even up to the company itself if it has been hacked.
Data is a very valuable asset. It can be sold to either trustworthy companies or less trustworthy companies. You could end up with a lot of spam or scam in your mailboxes.
Who ever reads all those hundred pages of AGBs and legal papers for each service?
If you would you might be shocked about what is sometimes going on behind the scenes.
What you do when nobody is around is also considered your privacy and if you think about it there are several reasons why you want to keep it like that. (read more)