Aerospace engineering. I design jet engines.

Every person is so specialized and every decision impacts everything around the parts/systems you own, you constantly need to be collaborating to make sure youre not screwing someone else. Or better yet, sometimes someone near your stuff can make an insignificant change (to them) that makes your life much easier.

I specialize in fluid and thermodynamics, so everything the air touches is impacted by my decisions... which is everything.