For many reasons.
1 - it might be very unique itself
2 - it might be very close to us
3 - it might be orbiting a very interesting star, a star type we didn't expect to have that planet type or so many planets, or so close, or so far, etc
4 - the more exoplanets we discover the more we learn how star systems come to be. the more we know how rare ours is, how it might have formed, etc. It helps us answer age old questions.