Another factor might be your transcript.
If you've taken a lot of poli sci, econ, philosophy, sociology, or international studies, for example (especially if you major in one or more of these fields) and are applying straight out of UG, it's probably clear that you're interested in law/government and not necessary to elaborate unless the prompt asks for it.
On the other hand, if you've taken a pre-med curriculum, or are a music or astronomy or math major, it would seem to be more important to give the adcoms some insight into why you're applying to law school, and that you're not just doing it because your parents want you to, or just because your boyfriend is applying.
This is an excellent point. Reiterating what one already has in their transcript (or in their resume) is simply redundant--best to use the limited PS wordcount to emphasize something else to make oneself a more attractive candidate. It could be good to explain why an BFA in ceramics might be applying to law, but for Philosophy or Poli Sci majors, the interest in constructing arguments, or in government, is implicit.