Flavour-wise, what is the use of an electric/gas BBQ?

When you're grilling food at high temperatures on a barbeque most of the smoke comes from fat that strikes the hot surface of the coals/vapourizer. It's only when you're slowly barbequing food, cooking it at cool temperatures, that the difference becomes important. With a gas or electric barbeque you'll need to add a source of smoke (eg. wood in a smoker box) if you want to add any smokey flavour. Even with charcoal it's normal to add wood to the coals in order to increase the amount of smoke.

When cooking on an indoor griddle or frying pan, unless something goes seriously wrong, there's usually no smoke so the flavour will different than when grilling on any sort of barbeque.