In my experience, I have had success in breaking down this question into 2-3 core reasons for the firm. Starting with 1-2 reasons (so maybe you like a firm for its PE and competition practices), identify why that practice is important to you (it might be they have a mandatory seat there or, alternatively, their unique position gives you the opportunity to work on advanced deals in that practice), then my 3rd reason relates to the culture and/or training at the firm (this could be focus on diversity, pro-bono, unique training contract structure), etc. that format has worked for me but ofc everyone approaches it differently.