One missing decimal point can send a tool crashing into a fixture, or blow an expensive ball nut apart with a runaway axis. Maybe your old-school machinists never make any mistakes, but looking at the posts here, there are few of them without mistakes in spelling, grammar and/or punctuation. That's in English, where it's relatively easy to spot them. Proof-reading a G-code program is much harder.
It's also a lot faster to generate code with a CAM program, or to change it if necessary. And you can tackle much more complex parts, which you'd never be able to code by hand.