Travel requires storage. In order to carry things with you, you need a place to carry them. My bicycle already had a decent aluminum rack (that I stole from my previous Mountain bike) that would soon carry a sizable bag to hold things like my wallet, phone, lock, airpump, and the like. While I prefer to travel light, there is always some stuff you need to carry.
This bag (pictures taken from Amazon) is a nice upgrade from the original Schwinn bag I had on my previous bike. With the fold down panniers, it holds more than you might expect.