You actually need to eat a lot of salt to survive, so it's much more of a trick than adding salt. MSG is not an essential nutrient that you need in any quantity, so using it is a trick to make your body think you're being well nourished when you aren't.

MSG is an essential nutrient. Glutamate (the G in MSG) is a component of protein rich foods, which we absolutely need to survive.

That's why we have taste buds for it.

MSG still gets you the sodium you need so it's not worthless from that angle.