From ibikeu Wiki

The 15mm wrench is one of the most used tools for cyclists, due to the standard use of 15mm nuts to secure wheels which do not feature a quick release. For fixed gear bikes, which generate too much torque in the rear wheel to be secured sufficiently by a quick release, a 15mm wrench is a must. Most urban fixie riders carry a 15mm wrench with them in order to remove their wheel to change a tire, or for adjusting chain tension.