Yes, it is an expression of submissiveness but its also a point that the WTS wants it adherents to dress up for the public,
this is suppose to make the organization stand out differently than ordinary Christendom and its followers.
Dress has always been a very important faction to the WTS., the outward image that they are trying to establish can probably go back
as for as Russell's door to door colportuers. In some ways its not any different to what a business wants to see of its employees,
when appearances are kept at a high level when dealing with the public.
This sort of dress code is simular with other religions as well,
such as Mormon, Mennonite, Hamish and the list goes on.
This not just exclusive to JWS.