Even if you can not use the card you get with "Sonic Bird", etc., right away, this prevents you from drawing it during the Draw Phase when you cannot use it. For example, if you do not have a Ritual Monster and Summon "Sonic Bird", you can get the Ritual Spell Card anyway. It will not be useful until you draw the Ritual Monster Card, but if you drew the Ritual Spell Card then you would not only have a card you could not use, you also would have wasted your draw to get it. Using "Sonic Bird"'s effect lets you avoid this. If you do not draw the Ritual Spell Card, you still benefit from having a compressed Deck.