It's probably so they can divert money into that corporation and keep it separate. That way, when they get sued, they can say "look, we don't have any money! Sure JW Congregation Support, Inc has money, but they are their own thing over there. You can't have their money".
This is Corporate Asset Management 101.
Eg. if a corporation leases out equipment, the equipment is owned by a separate corporation than the the corporation that does the lease agreements.
An extra layer of insulation against lawsuits.
JW Support only OWNS the buildings, and leases to the congos.
What takes place inside is the responsibility of the congo, not JW Support.