Right, it's optional, but their interview is strongly recommended according to their website, and the admission officer who interviewed me mentioned this as well.
The possible function of their interview is for them to get to know us in detail.
Although the information on our applications represents our personality, academic accomplishments, and work experiences, I bet they probably want to listen the human answers from us. I have low speaking score, too. But I think I appealed myself very well through the interview like mentioning my background and career objective. especially, I told her that I started reading Bar exam materials, and she got surprised about that. haha
That's why I interviewed with them.
It's up to you.