This question is designed to demonstrate how much research you’ve done on the firm as well as to see if you might be a good "fit." To get further information about a particular firm, you should read recent press stories and visit its web site. This answer should be based on your actual reasons; you don't want to get caught in a lie. You should still manage to show that you know a bit about the firm, its people, its culture, and its specialties in your answer.
Other things to know and weave into your answer include: Is the firm a small firm and ostensibly hoping to stay small or trying to get bigger? Is the structure flat with few layers of management or are there several titles between analyst and managing director?
As an example answer, you could emphasize the people and discuss why you like the people you've met and why this makes you want to become part of the team. Note that while it is good to talk about the firm's people and culture, it is not the best idea to blatantly state that you want to work for a firm simply because it’s prestigious (for reasons similar to why you should not discuss wanting to make money).
Like many interview questions, there are lots of potential subtexts that might start running through your head when you hear this one come out of an interviewer's mouth. Are they trying to get me to dish the dirt on my current employer?
This is question you are almost guaranteed to receive in any interview. And although you may think it's a softball question—one easily answered by spouting out a few facts gleaned from a Google search—this couldn't be further from the truth.