They probably want access to all your contacts with their phone numbers and email so they can build better graphs about your social circle and build shadow prof

They probably want access to all your contacts with their phone numbers and email so they can build better graphs about your social circle and build shadow profiles on all your friends. If you go further you can probably find hundreds more interesting data points like constant location tracking or full access to any media file mixed with face recognition software. In short: Your phone is a goldmine of information not only on yourself but anyone you know.