Why do you add couscous to boiling water? All recipes for couscous I've seen instruct to add the couscous to already boiling water.

Why is that so? What difference does it make if it will be added to boiling water or cold water on the stove?
You don't really 'cook' cous-cous, it's already cooked - you just need to rehydrate it sufficiently, without getting it too wet &amp; going sloppy.

Starting with boiling water give you a better time-reference - 5/10/15 minutes from that known start-point, depending on the specific cous-cous type. You switch off the heat as soon as you add [or in many cases just pour water onto it fresh from the kettle then let stand.]

Starting from cold would introduce an unknown variable - how long does it take to boil?