Hmmm, happy to be told I'm wrong I've been advised before not to link to the firm in this question haha, unless explicitly relevant (i.e a previous scheme of some sort).
In my experience firms that ask this are looking for personality, and a full picture of you. They will almost certainly ask you some variation of why law and why this firm in the rest of the interview/application process, this is usually a bit of an icebreaker to give them an overview of who you are.
I generally go where I'm from, where I went to uni and what I'm studying. Then I move on to societies and personal interests (which for me combine as they're mostly sports and non-academic). And I'll mention a bit about career aspirations and work experience to date but again not like I'm answering "why law" all over again. For me this might take the form of "I've loved my stem degree but after doing X and Y scheme, I've realised my interests are more aligned to a law career" and I always mention my hospitality experience as it's the most significant part of my application usually.
Ultimately you're working to make yourself sound more interesting and different, what is your USP? If you go in and say "I'm a law student at X russell group, and I was the treasurer for the law soc, and I want to do corporate law, at a corporate law firm" that tells them nithing about you that's not on your CV, and makes you seem cookie cutter with not much going on.