the common theory is that writing about why you want to go to law school is only a good way to go if you have a really, backed up by years of commitment, good reason. Otherwise, the PS is a chance for the admissions C's to see a person behind the numbers, so give them a glimpse of why you are different from teh thousands of others apllying. Anything that makes you a little different is good game. Without knowing more, I can't help more specifically.
GL!