Project Activity is just like an audit trail. Almost everything is captured in the timeline.
Some of the highlights are important things like the sending of an Invoice. You be able to see the User who sent it, the date it was sent and the message that went alone with the email.
You'll also be able to see when someone Deleted something. Almost as important as when something is created. Regarless you'll always have some record of key important events that take place within a Project.