If they're a compatible architecture (protocol and bus) the wind indicator will be able to get speed information and do the speed/angle calculations so you alternate between true and apparent wind display readings. If you get a "stand-alone" wind display you will only be able to display apparent readings.

I only use apparent readings when making reefing decisions on a reach or beat, but off the wind or on a deep reach it's really nice to be able to flip over to true. If you are a racer you will want true wind indications for various decisions and it will give more accurate readings than you can estimate by reading the water.