quick timing belt question.

[chronic139]

I started thinking today that I want to change my timing belt. Theres no rush on this but I would like to do it for the experience if nothing else. Can someone tell me, what is the purpose of the cam hold-down tool? Do the cam sprockets need to be loosened, or could a person basically just re-route the new belt around each sprocket and pulley without moving them? just out of curiosity. Thanks