Remember when you were a kid and there was a birthday party at the pool and your parents totally embarrassed you by slathering you all over with sunscreen and you were all "MOM I HAVE ENOUGH SUNSCREEN" and she wouldn't listen? Well, if you had this UV Index sensor connected up to an Arduino you could have said "According to this calibrated SI1145 sensor from SiLabs, the UV index right now is 4.5 which means I do not need more sunscreen" and she would have been so impressed with your project that you could have spent more time splashing around.