Series 7/66...what does adding/not adding a 79 entail?

Licensed 7/66 - for M&A-IB, why would one need a 79 if it is a 'carve-out' of the 7? Assuming your bank doesn't require it, or if you were to partner up to create your own BD, do you have to add a 79 when already registered with a 7? I've searched brokercheck on dozens and dozens of IB MD's I know and most are 7/63 (or 7/66). Will come across a 79 every now and then, but it's an anomaly - usually more jr. in those cases.