Marking the belt does one super important thing. It makes absolutely sure that you have the EXACT proper amount of teeth between the two cam sprockets. Because of the tension that is released when you take the belt off, when the belt goes back on, the belt is off by one tooth all the way around. By marking the belt and sprockets with a silver pen, when it all goes back on you line each up and make 100% certain that you are in correct placement with that belt on top, which in turn makes the rest if the belt 100% the same as when you took it off. It is the easiest foolproof way of getting it done.

Otherwise, you're just hopeing you have the proper amount of teeth. Chances are, to get that belt back on correctly on the cam gears, youll need a wrench to spin one cam gear a bit to line it up. Getting the top right makes the rest fall into place exactlty like you left it.