If you are doing the long tail cast on and don't start with enough yarn tail, you will not have enough to make all the cast on stitches, since they are formed by the tail yarn, not from the skein or ball of yarn.
If you DO run short, you can always add a few with a knitted cast on.
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Basically it's so you're not wasting a lot of yarn for a long-tail cast-on. You can measure off what you need, have a bit to weave in later, and have the rest for your project. If you happen to be using a type of knitted cast-on, then this method doesn't apply. Different projects may use various cast on methods (some may need a firmer or a looser edge), so it helps to know some tricks for each.