A fair piece of the money flowing to NAD from various conferences and other institutions, funds medical and retirement benefits for denominational employees in North America.  These functions are much more efficiently handled on a nation-wide basis.  And the Tithe subsidy is the tip of the iceberg in terms of the total cash flow for these functions.  Conferences and other institutional employers also kick-in a % of their payroll.  And a fair amount also gets deducted from employee paychecks.

Then there is another big chunk that flows-back in the form of subsidies to educational institutions and another chunk for nation-wide media ministries, etc.