So I just started the game (at the lav fight) but I was just curious about the affects of the combat unit. I know the two other ones I have now play a significant purpose but I'm confused on combat staff. Thanks for the help

Eventually you get to send your combat unit on ops of their own, then you see the results after your mission. If you capture tanks, helicopters or whatever you can use those too.

I think if your combat unit's level is high then you also unlock new guns.

Eventually you will have five different units: Combat, R&D, Mess Hall, Medical and Intel. Combat can be used for Outer Ops missions and adds BNP (or whatever the money is called) to your pile so you can research new gear through your R&D unit. Also, for some weapons your unit's total level will have to be at a certain point.

You can take units from the Combat Unit and play as them in game if you think they have similar or better stats than Snake. Or if you play co-op and someone else takes Snake instead.

Cool this has been really helpful. I found it weird that it's open from the beginning but never explained