|Funakoshi Gigo in Gi|
Hakama are even worse. Although there have been some martial arguments made for wearing hakama, they were originally based off of leather riding breaches used during medieval Japan. Why do I need to wear them to do Aikido? I don't wear leather chaps when I fire my sixshooter at the range (though it would be fun to see someone do that). There are several reasons why we do, and I'll discuss some of them, and then tell you why I think we wear them.
There is a certain amount of functionality built into the traditional martial arts clothing. They keep us covered, they are made of durable material (thicker cotton or canvas), they are firm enough to keep up with the abuse we put them through, and they absorb sweat relatively well (at least the judo gi that I use). The downside is they are generally more expensive than workout clothes.
It is traditional
Yes, there is a lot of tradition associated with martial arts, and the clothing associated with them is part of that. Originally, the instructors were people from some Asian country, and in that culture, these clothing would be appropriate. However, I am a white guy from Wisconsin who has never even been to Japan, let alone have good grasp of the culture. Why should I wear something that isn't culturally significant to me. Well, it is basically because my sensei did. Why did my sensei? Because his sensei did, and so forth. There is a connection to these other people, to all martial artist because of the uniform. It is a recognizable symbol, and in that, there is value.
The Ritual Reason
The Sacred and the Profane: The Nature of Religion by Mircea Eliade