Why is the Alignment step necessary in HDR?

Question

A basic step in HDR (I guess) is aligning the bracketed images together, but what I don't understand is why we need this step. The images are all the same, since they were taken from the same location (maybe on a tripod) with time lag of only a fraction of a second. So, it looks like all the images have the same settings except the exposure. So, what is alignment all about?

Answer

With high resolution images, even the tiny-est vibrations will be visible in the image when looked at the pixel level. The same holds for slight movement of the subjects in the image (from wind, motion, etc.). Misaligned images degrade the quality of the result of the HDR processor.

Note that if you shoot RAW, you can use a single image to generate a few bracketed jpegs of different exposure levels. These jpegs are perfectly aligned, so if the dynamic range you want to encompass in your HDR image is small enough, you can use these aligned images as input to the processor.

Check more discussion of this question.