Hi there,
Great question - you can definitely use the same structure or follow a 'PEE' (Point, explanation, evidence) kind of structure. You can start broad, and mention what draws you to law in general (e.g. analysing complex problem, helping clients achieve outcomes, etc..). However, you should definitely focus on commercial law if you're applying to a commercial law firm, and your reasons to join might be different than just for law. I think you should definitely include the major features of a career in commercial law and how it appeals to you. For example, frequent client interactions, working on major cross-jurisdictional transactions and deals, fast-paced nature of the work, mixing law and business... For each point you make, definitely mention how it suits your interests and your strengths.
Hope this helps!