Because you think the company is a good fit for you.
Of course then you have to follow up with why - which means you did your research on the company and job position.
Nobody wants to hire somone who will be miserable at their job. It costs a lot to look for, hire, and then train an employee -both in time and money. If a company goes through all of that and then is misearable and leaves for an other job, then nobody benefits.
Does the job invole travel? Does it involve meetings with clients? Is it a team based role or an individual contribuitor, where is the office located, what sort of commute would you be looking at, what sort of hours are expected of you. These are the sort of questions you have answered before the interview (to the extent prossible)
Then when asked the "why do you want this job" question you can say things such as "Not only do the technical skills required for this job meet my skill set, but I see this job requires making presentations to prospective clients. I enjoy public speaking and presenting, and in fact I have been in Toastmasters for the last 3 years. I also see that the job requires some international travel. This is not an issue for me, in fact I would enjoy it. I have traveled to a number of countries and I speak Spanish, and a bit of French, and would be interested inlearing other languages if necessary"
Of course whatever you say needs to be true and spun to make you look good. If the job requires long hours and working alone, don't say " This would be a good fit because I am an anti-social, socially awkward, hemit". Say "This would be a good job for me because I noticed in college that while working on a number of overnight projects in the lab, that I did not mind working odd hours by myself, if fact sometimes I had some of my best insights into problems at such times"
It's all about how you present yourself.