Uh it really applies to all those fields. Want to work at a good law firm, then you have a better shot if you do well at a good law school. Want to work for an ib bank, in pe, consulting, tech, engineering (fields that pay money) then you need good enough grades to at least get your apps looked at.
Any top school, law firm, hospital, or company gets thousands upon thousands of applications each year. If you want interviews and a decent chance you need good grades.
They know just like the rest of the world and the genius who pointed it out above that you won't remember what you learned in college, but they need someway to distinguish candidates that is cost effective, i.e. how you look on paper. That's not to say your life is over, but it can make easier once you enter the real world.
You can always work your way up or you can invest in your future and not start at the bottom.