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The card Cryptbreaker has the following ability:
Tap three untapped Zombies you control: You draw a card and you lose 1 life.
Why is it necessary to specify that the three Zombies must be untapped? You can't tap an already tapped creature, or use tapped creatures to pay a tap cost. MtG cards usually have a very good economy of words, so why include this seemingly unnecessary text? Would the card function differently in any circumstance if it just said "Tap three Zombies you control"?
You are correct that the word is unnecessary; it would function the same without.
There are various times that MTG adds extra words for the sake of clarity; even when they have no effect on functionality.
A very common example are cards such as Aetherstorm Roc, which use the wording "Aetherstorm Roc or another creature enters the battlefield under your control..."
This could have simply been written "Whenever a creature enters the battlefield under your control" for the exact same effect. But there would be some less experienced players that would not know if it should count itself or not. The longer wording makes this explicit.
"Tap an untapped creature you control" works the same way. If it simply said "Tap a creature you control", then you wouldn't be able to activate it without actually tapping an untapped creature, due to the rule:
701.20a To tap a permanent, turn it sideways from an upright position. Only untapped permanents can be tapped.
However, the extra word helps make it clear that this is the case. Especially due to the fact that a "tap target creature" ability can in fact target a tapped creature, some people would wonder what stops them from choosing tapped creatures to pay the activation cost for "tap a creature you control".
If I had to guess, I would say that it has to do with the fact that yes, you can't tap a tapped permanent to pay a cost, but if a spell or ability has an effect that taps a permanent, you can target a permanent that is already tapped, depending on the wording of the spell or ability.
A rule (I don't know which one, I'm not in a place where I can look up the number) says:
If an effect attempts to do something impossible, it does only as much as possible.
For instance, Hands of Binding is a blue Sorcery that says:
Tap target creature an opponent controls. That creature doesn't untap during its controller's next untap step.
Cipher (Then you may exile this spell card encoded on a creature you control. Whenever that creature deals combat damage to a player, its controller may cast a copy of the encoded card without paying its mana cost.)
Notice: It does not say Tap target untapped creature an opponent controls.
Let's say that your opponent has two creatures, one of whom is tapped; you have one creature. You cast Hands of Binding on your opponent's untapped creature, then cipher it onto your creature. You attack and when it deals combat damage to the player, you get to recast Hands of Binding. You can target the other creature even though it was already tapped.
When you (re)cast Hands of Binding, the creature is a legal target, because the only requirement is that the target be a creature. When the spell resolves, the target is still valid (the spell checks for legality twice). So the effects start to happen.
The first one tries and fails to happen because, as has already been mentioned, you can't tap a tapped creature, so that effect fizzles.
HOWEVER
Then the second effect happens:
That creature doesn't untap during its controller's next untap step.
That effect can happen, so it does. Presto! You now have two creatures that your opponent controls that won't untap during that player's next untap step.
I think that it's specifically this type of interaction that leads to the "extra* text on Cryptbreaker and other cards like it. Technically it's redundant, but it alleviates the confusion for people that are familiar with this interaction.
