The purpose of this is to clarify the boundries between the state and the individual. The state should not make decisions about what is proper or attractive or fitting. The state should only get involved when our behaviour can have an negative impact on other people.
If someone is acting in a violent way, innocent people may get hurt. If someone is creating alot of noise, other people can suffer in all kinds of ways. But if someone has a haircut or t-shirt other people don't like, they can simply look elsewhere. If someone is wearing too much, or too little, for someone else's taste, they can simply look elsewhere.
There are valid reasons to limit what people wear, but the fact that someone else does not like their choice is not a valid reason. In a free society, only necessary limits on our freedom should be permitted.