These allow you to craft more cards you might desire, thus converting into more cards for your collection
The other 6 card types are "permanents", which means that when you play them they go onto the table in front of you (known as the "battlefield" and remain in play until something moves them somewhere else
Admittedly, most of my experience casting this card was in my Goblin Charbelcher glory days
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However, McDonalds has also stated that, while the official policy is to not sell Happy Meals promotions separately from the meal, some restaurants may "accomodate reasonable requests"
There are a few key differences, like that split cards have the combined mana value and types of both halves, while MDFCs only have the properties of the front half when anywhere other than the stack or the battlefield