In the context of medical imaging, these regions have to be anatomically meaningful.
A typical example is partitioning a MRI image of the brain into the white
and gray matter. Since it replaces continuous intensities with discrete labels, segmentation
can be seen as an extreme form of smoothing/information reduction.
Segmentation is also related to registration in the sense that if an atlas15 can be
perfectly registered to a dataset at hand, then the registered atlas labels are the
segmentation. Segmentation is useful for visualization16, it allows for quantitative
shape analysis, and provides an indispensable anatomical framework for virtually
any subsequent automatic analysis.