The argument goes something like this: The Bible says that where two or three are gathered there is Jesus in the midst of them (Matt 18:20). That indicates that a gathering of two or three includes the very important presence of the Lord Jesus Christ. People wear their best clothes when they meet someone Important (like if one had a meeting with the President). This indicates that the esteem put on the person justifies the better clothing. Christ is more Important than anyone and to be greater esteemed. Therefore, when going to any meeting of the church a person should wear their best clothes.