Dry Shampoo allows you to skip a day or two of shampooing. Eden and I use this constantly when we are traveling. If you are at a hotel that doesn't have conditioner the last thing you want to do is shampoo your hair.
We prefer the dry shampoo drybar makes, it has a super-absorbent formula that eliminates oils, dirt, and impurities in between washes, leaving hair fresh and clean. Plus, it smells really good.