Every time you perform a draw which has the image of Mileage on top of it, you obtain the displayed amount of Mileage.

Usually, an X1 draw gives you 1 Mileage while an X10 draw gives you 11 Mileage.

You can Use Mileage by going to Shop → Item Exchange → Mileage

As you can see in the above image, you can purchase certain card packs using Mileage.