Alignment compensates for tiny shifts between bracketed frames. Even if you shoot from the same spot, very small camera movement can occur from tripod vibration, pressing the shutter, mirror/shutter motion, or simply using less-than-rigid support. At pixel level, a shift of even 1–2 pixels can reduce sharpness or create artifacts when the HDR software merges the images.

Alignment can also help when parts of the scene move slightly between exposures, such as leaves in wind or other motion, though moving subjects may still require separate ghost-reduction tools.

If your setup is extremely stable and nothing in the scene moves, the images may already be aligned well enough and some software lets you skip that step. Another case is when you generate multiple exposure versions from a single RAW file; those derived images are inherently aligned because they all come from the same capture.

So alignment is mainly a safeguard: it ensures the bracketed photos line up precisely before HDR merging and tone mapping.