In 50 words, I would state one thing about the firm's ethos, the TC structure/job profile, the type of clients and deals and/or the career opportunities available that stand out to you. There simply isn't enough space for you to explain why these things stand out to you so you're probably better off just stating what stands out to you plainly. Alternatively, you could pick one headlining thing about the firm you like and use the remaining 50 words to explain why you like this aspect about the firm.