The BfSB is just the old corps MI BDE with shift toward a HUMINT/CI focus and away from many of the 'big' systems like GSRs. There's no significant change in the mission from when they were corps MI, but as a part of all the other terminology changes (what's the actual difference between a "brigade" and "brigade combat team") we just had to have one more.

As to state missions, unless you're expecting mass riots somewhere that need a large influx of troops to augment the local PD (a la LA '92) you'll be fine with a BfSB.