Justifying one thing because it's a necessary component of another *unnecessary* thing... what logical fallacy is that?

That's old fashioned rationalization.
Doesn't necessarily have to be a logical fallacy

Can you elaborate?
- I can provide an example, but you might hate me for it. I swear this is just to explain what I mean by this fallacy because I can't think of another example right now.
  Justifying killing/using an animal for its skin/hide (e.g. leather or fur), because you're already killing the animal for its flesh, when in actual fact the killing of the animal doesn't need to take place at all (hypothetically).
  Or justifying the killing of calves for veal as a necessary component of dairy production, when in fact dairy production isn't necessary, either.
  I hope that makes sense
  
  I don't think it's a fallacy, just really mixed up.
  
  I'm going to come up with a neutral one here for you. Mostly so you can get better replies... I would need to do some research to find a name for it.
  Jess wants to draw a picture of a bird, using only red and black. She gets her art supplies out and finds she has no green markers. She cannot draw a picture of a green field without a green marker. Jess says she can't draw her bird picture because she has no green marker.
  Or...
  When Alan plays tennis, his knee hurts. Alan has a strange condition that his knee only hurts after he plays tennis. When his knee hurts, he has to put ice on it. He stops playing tennis for good. Alan needs to get a new refrigerator. One is affordable with no ice machine, but another is unaffordable and has an ice machine. Alan bemoans he can't get the refrigerator he needs because he needs the ice machine for his knee.
  One of these?
  
  Neither of those are good examples. The killing of calves is not necessary for dairy production, they could always be sold to be raised to adulthood which is what happens to most of them since that's more profitable.
  And the using of the animal skin/hide IF you're already killing it for food is a perfectly valid argument for using the skin/hide, the alternative being killing the animal and disposing of the skin/hide. You might then shift your argument to attack the need to eat the animal which is another (and a lot more complex) discussion, but the initial of why using the skin/hide is solved to both parties satisfaction so you reached a common ground, i.e. you agree that IF one were to kill the animal for food, the use of his skin/hide would be acceptable, now the argument needs to shift to whether it is acceptable to kill the animal for food.
  If then the person argues he's killing for food because he's already using the skin/hide then he's using a circular argument. If he can provide an argument you consider acceptable for eating the animal you would also agree that it is acceptable to wear him.
  
  I know it's just an example but I wanted to point out that almost no calves end up as veal.
- I need sour cream for my tacos, and I need tacos.
  
  What where's the "unnecessary" part?

I think you mean false premise or argument from false premises. "A, therefore B", even though A is false.

Post rationalization perhaps?
It could also be similar to the logic of phrenology, where they pointed at objective measurements of various features of people's skulls to make wild conclusions about the superiority or inferiority of different races. It's not that they were falsifying the measurements, they just skipped a step and treated the sensibilities of the most privileged and influential members of society as objective fact.

Unnn... the whole is greater than the sum of the parts?

Logical fallacies do not involve themselves in questions of worthiness, usefulness, goodnesss, etc.
If the fallacy relies on this pattern of detecting necessary and unnecessary, then it is not a logical fallacy.
- I think they can certainly apply to any situation where the logic is flawed, so arguing that something is necessary since it's part of another system which itself is unnecessary, is a logical fallacy
  If we accept that something is a necessary component of an unnecessary system, but then use that fact to argue that the component is necessary in absolute terms, that's a logical fallacy given that it's not absolutely necessary if the system it's a part of isn't absolutely necessary
  After researching I found it can be called a false necessity fallacy or false requirement fallacy
- But the term you’re looking for is tunnel vision.
  It refers to committing resources to interpretation of the world around a single immovable assumption.