Why are there 3rd party rules?

http://arrl.org/third-party-operating-agreements lists the countries we are allowed to pass 3rd party messages from.

The fcc regulations specifically state that 3rd party messages must be of a personal or emergency nature.

Why are these rules in place? What is the functional purpose of them? If 2 hams can pass todays baseball game scores to each other why can't we send a similar message on behalf of a 3rd party?