Bench bleeding is usually only necessary with a brand new master cylinder. Even with a complete fluid change, you'll never get the master cylinder completely dry like they are when they're new.
But, when the ARE new, a bench bleed may be required because you might not get enough piston travel in the car to eliminate all the little air pockets in the master cylinder. Sometimes bleeding in the car works fine, sometimes you'll chase your tail for days and still have air pockets in the master. Bench bleeding is recommended with a new master for that exact reason.
