Dice are random.
This sounds obvious, but they are about the most random thing that can be added to a game. Every roll has an equal chance of happening, every time you roll the dice. There are hot streaks, and poor streaks... and sometimes the dice just seem to have it against you.
Dice cards give you more control over the level of randomness in your game, as every possible roll is arranged as part of a deck. You are guaranteed to get an even distribution of rolls - unless you draw the shuffle card.
This card keeps the deck from being totally predictable - by shuffling the deck before you reach the very end, you're not going to draw every number from 1 to 20 each time around - but your runs of bad luck will never be quite so bad.
Included in dice cards are printable cards for each of the standard polyhedral dice: d4, d6, d8, d10, d%, d12, and d20 - a seperate file for each.