It's helpful if given a partition (with sharps or flats duly noted in the key signature), of which you want to quickly know the key -thus the tonic chord, usually the one any given song begins and ends with... broadly speaking. With this mnemonic you can immediately tell what the key is and start playing. Of course most people who play an instrument have developed their ear so that they can find the key of most songs they hear intuitively/empirically, the trick is useful with a partition only.