It is essentially a legal requirement for the USA, although other branches do this too.
In the USA, all money received as a stipend from the borg is tax free. Ditto the food and shelter that is provided. To receive this tax free status, a religion must qualify those who work full time for a religion as belonging to a "religious order" such as priests in the Catholic Church.
There are a few religious orders used in the USA by the borg; one for Traveling Overseers, one for Bethelites, one for Assembly Hall personnel, another for American missionaries. Even though they all have to file a tax return each year, they don't have to pay any tax because of this system. And they have to sign a document like this.