Speaking from an engineering point of view,I always like to have some-one on the headset during engine runs.I presume for the same reason's a flight crew have.
You can't see the engine during start so first of all you need to be aware you are clear to start ie. no-body or nothing in the way.It's also handy to know there is positive N1 rotation as it can be seen visually before it's indicated in the flight deck.
Also,being in the flight deck,I would be completly unaware of a fuel/oil leak gushing out which I would particularly like to be aware of!