Palmprint Identification - Why do we align the images before we use the Fourier Transform?

I was reading the paper PALMPRINT IDENTIFICATION BY FOURIER TRANSFORM by WENXIN LI, DAVID ZHANG and ZHUOQUN XU about identifying persons based on an image of their palm.

• The first step in their identification algorithm consists of aligning and pre-processing the image since they spent a few pages on that front, this gave me the impression, that this step is kinda important.

• What I didn't understand however: Why do we do this? The essential part in my opinion is the Fourier Transform afterwards, that is used to extract/generate the feature. But from my understanding, this should even work without this aligning/pre-processing step.