The rebels can be attacked twice a day and some of their rewards include beast scrolls and chests. Almost the only way to obtain beast items for free other than the alliance daily tasks sometimes rewarding skill chests.
The rebels need to be done whenever possible.
To achieve the next level of rebels you only need to exceed the combat power of the defenders (as long as you have a good attacking setup) see solo army once its published.
These are the possible rewards from stage 10 of the rebels, you do not get all of the rewards each time.