Hey!
For this question, I would approach it in two clear stages - first define what you are genuinely looking for in your legal career, and then link each of those points directly to something specific about the firm.
In the first point, make sure that you use genuine and concrete examples of what you are looking for. Instead of saying things such as "high-quality work" or "a challenging environment" (which could be seen as quite generic), you could discuss things such as early responsibility, sector specialism, or a collaborative environment where you can work with lots of different people. You can then frame this around the kind of lawyer you want to become (e.g. technically excellent or commercially minded).
Then, for the second part, show how the firm would be the best place to achieve that. This is where you need to show that you have researched the firm and understand its strengths - you could reference its training style, client base, culture, or particular practice area strengths. Just make sure to draw a clear line between your long-term development goals and what the firm actually offers, rather than discussing the two separately.
A simple structure for this might look like:
"I am looking for X because I want to become Y. A firm like [Firm] would support this because of Z".
Best of luck!