This question exists because it's hard.
One reason it's hard because the true answer is always: "because you believe that I will complete more work and therefore generate more profits than the other 8 people you interviewed, and because you like me more than them based on the 40 minute conversation we just had." That is not a valid answer in most interviews I can imagine taking place.
Another reason this question is hard is that questions which ask you to boast about yourself are always awkward to answer. I find it always comes off better to be humble in interviews, and this question seems like it's almost begging you to talk yourself up.
Given this, I would approach this question by ignoring it (more or less) and answering a different question: "Why do you want to work here?"
"This area genuinely interests me and I'm excited about this work."
"This is the work I want to be doing. I prefer the work that goes on here to the work that goes on in the 3 other places I interviewed at last week."
I think you can leave out the "...so therefore I will work very hard" bit, because that's a given. Convince them you actually want the job. If you succeed, that's a valid, though roundabout, way of giving them a reason to hire you.