They have both in-house counsels and work with external law firms. A reason for doing this is to avoid liability.

Some big companies retain multiple law firms and ask the same advice a couple of times. If an overworked attorney who barely gets any sleep ever misses a point or drafts his/her advice vaguely in a way that would enable their evil plans, they rely on that single opinion. If there are any problems they don't assume any liability, because hey they got a legal opinion from a respectable law firm. Nobody needs to know/can prove they obtained multiple opinions on the same matter because of legal privilege.

In return, big law gets paid a lot of $$$, because of the risk of providing such opinions under considerable pressure and trickstery.

Having a legal opinion supporting an aggressive tax position that is eventually overturned doesn’t get you out of liability for that position. The appearance of food faith efforts to comply with the complex tax code may reduce penalties assessed.

Sure, my comment wasn't tax code specific. There is always risk involved, but it's much less. If you need to do it anyway, better pressure a lawyer into giving you a legal opinion. The company will pay pay the fine but who can blame you for acting on it.

In a big corp, I think most employees are concerned with saving the day. If the management pressures them to cut corners most will. What better way is there to pass the blame?