4BLD question

In 4BLD you sometimes have to swap to B or D for centers, even though they are both just white centers and colour-wise, the algorithms are the exact same as doing a U2. Is there a reason this is necessary?
Which letter scheme do you use? I use the one where the top left is A, the top right is B, and so on. I also have A as the buffer and C as the target. If you use that, swapping to B and D is necessary to get a non-white piece to the buffer. (Not sure if you already know this, but if B or D comes second in a letter pair, you have to switch to the other one.)
 
In 4BLD you sometimes have to swap to B or D for centers, even though they are both just white centers and colour-wise, the algorithms are the exact same as doing a U2. Is there a reason this is necessary?
You have to switch B to D or D to B because each alg you do (with the U2 interchange) swaps them. I cycle from B or D (might be swapped) to C to A back to B or D and do a U2.
 
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