Traditional martial arts are known for the use of a training uniform, the "gi." The founder of Judo, Jigoro Kano is credited with its creation along with the belt ranking system (kyu - lower grades and dan - black belt grades). The keikogi (kay koh gee) or dogi (doe gee) is actually an augmented under garment similar to a pair of long-johns that is designed to withstand the rigors of martial arts training.
For example, judo students wear a quilted gi meant for being grabbed and pulled because of the many throws and joint locks in the art whereas a karate gi may be single layer of cotton or and a bit less constructive to allow for punches and kicks. The jacket is held together with a belt that represents a students rank and status similar to pre WW II Japan.
There are a growing number of marital arts schools that do not use a gi and prefer to have students in shorts and t-shirts or street clothes because it allows for greater movement as well as a more "realistic" approach to fighting. And while it's true that the gi has only been around for a little over a hundred years, there are some great benefits.
Mental Focus. Anyone who have served in the military or is or has been a first responder understands that once he or she dons the uniform, their focus must be on the immediate task at hand. This is also true of traditional martial arts uniforms. Wearing a gi is a reminder to everyone (in the dojo) that the environment is one of mutual respect and discipline.
Cleanliness. People sweat a lot. A gi absorbs a lot of that sweat (and blood) that would otherwise end up on somebody else. Of course, those of us who practice grappling arts accept that fact that we are going to swap sweat with other people, but a gi will keep this to a minimum and help to keep the training mats from getting overly saturated.
Discipline. One of the most important aspects of traditional martial arts is discipline. Depending on the type of art you study, there is typically a formal way to wear, fold and care for the gi. In traditional Japanese martial arts such as aikido, jujutsu, Iaido and kyudo, the hakama or pleated pants are a part of the training uniform. The process of folding the hakama after class is just as much a part of the martial discipline as the art itself.
Our students wear a gi for training in our dojo for the majority of our classes. Some times we will have class with t-shirts and sweat pants or street clothes just to mix things up a little bit. One thing that I've noticed over the years, having had classes with and without traditional uniforms is that attitudes are visibly different when folks are not wearing a gi. What do you think?