Example 1: Probability of lands in first four turns
If you want to know how what the probability of playing lands in each of your first 4 turns (you don't go first, 4+7=11 cards drawn), in a deck of 60 cards where 20 are land. Try it! If you want better than about a 50/50 chance of playing lands on each of your first 4 turns (to get that Juggernaut out), you need to add more land, elves, dark rituals, or something similar.
Example 2: Probability of drawing killer card within five turns
You have 4 I-win-you-lose-super cards in your deck. What is the probability that you'll draw one of your super cards in the first 5 turns? (5+7=12 cards drawn. As before, other player draws first so you get to draw on turn 1, 60 card deck). Try it! You can improve your odds by adding impulse, intuition, or something similar.
Example 3: Probability to draw and play a WW Creature
You are playing a 60-card white and red deck with 13 plains and 9 mountains. Your deck is loaded with WW creatures. What is the probability that you will be able to bring out one of these creatures on the second turn? Not good enough for a fast deck, you'd better put in more plains!! With Deck-u-lator we can also factor in the probability of drawing a WW creature to go with those 2 plains by adding a third card type.