Wanting to become a commercial lawyer is one thing but what the firm can offer you as a person in your career is quite different. For a start, one of the key things in this question is what they as a firm specifically can grant you - they says 'a firm like ours' but definitely try to tailor this. Therefore, you want to be thinking about what makes Debevoise different. I personally cannot speak much for the firm as I have never applied or interacted with them but for Freshfields, for example, they have a great focus on mentorship so if I was writing this with them in mind, I might draw this out and discuss what that mentorship can offer me as a person and offer some more detail as to why I think that matters.
Another thing you can discuss would be the international opportunities you might gain - but as a word of caution - ensure this is unique to the firm as realistically this could be said for any international firm in the city. Does the firm have a particularly strong focus in any areas in particular?
Try not to overthink this question too much - just have in your mind what you are excited about for your future career and the experiences it will afford you and just try to keep it really tailored to Debevoise themselves.
I hope this helps.
