Most Miracle decks, regardless of their colour combination or format, have similar core cards – or rather cards with similar functions.
Because Miracles care so much about being drawn as the first card in a given turn they require proper deckbuilding so that this restriction is mitigated as much as possible. This can be accomplished by playing library manipulation cards and instant speed draw.
The answer is simple – otherwise these cards are stranded in your hand and you have to cast them for their full cost. It’s no surprise that Miracle cards are overcosted if they are meant to be cast naturally. If you want a consistent deck, you need these cards.
Moreover, there is much more to it. Not only do they allow you not to sit there with a bunch of uncastable spells but also to monitor the timing. If you can decide when you put them on top, you’ve got much more control on the timing. You can even put them second from the top to sculpt the perfect subsequent turn. But don’t forget which position the card is in!