As said it does add the "coolness" effect to the jutsu, but there is also something else to it...

When u say or scream out something when u do a move (or whatever) u are able to put ur mind to it and thus making such a move more powerful...

It's even so in real life..
Like, when u do some heavy lifting u can give just that bit more power to lift something up when u scream or make some other loud noise...