Why do law journals require a CV when submitting a paper?
I'm an occasional reviewer for a prestigious law journal in Canada. I mainly deal with articles that talk about data science in the law field.

Now, you're more likely to listen to my answer specifically because I stated my credentials. I gave you a reason to listen to me: I'm an authority on the subject. The main reason they ask for your CV is to give you an opportunity to do the same.

In my case, if the author doesn't have anything in his or her resume that approximates professional experience with data science, I'll pay closer attention. This is because, in my experience, the person is more likely to try to sweep his or her lack of understanding of a specificities under the rug. For example, he might talk about p-values and draw conclusions based on them, but then fail to mention the numerical values and/or the interpretation context (i.e. fail to define "highly correlated", etc.). 

In my opinion, the CV is an intuitively valuable data point for the reviewer. Just remember that it's one piece of information out of several.

EDIT

I want to add that the CV alone does not and should not change the outcome. As a reviewer though, if you're talking about tech and I see that you have no relevant experience in it whatsoever, it helps me give you better feedback. A lot. I regularly take the time to give thorough explanations in my comments and the depth of my interventions will vary based on the CV. If you have experience in say statistics and you mess up p-values, I'll rip into you. It's that simple. Don't waste my time when you should know better. If you have no experience and make broad generalizations that turn out to be false, I will also rip into you, but I might be generous enough to offer general advice. If you have no experience and you make a slight mistake, then I'll be kind and helpful even if I don't accept your paper right away just because you're from Harvard. I'll provide links, book titles, articles, etc.