As far as I know they started them on the EJ25D. Subarus first Interference engine. So I think they wanted to make sure it wouldn't jump. I see lots of early engines without them. Some the oil pump still has the holes, others they aren't there.
It's just that Manual trans the engine is much more likely to go from fully loaded pulling hard to 5500 rpms or more, then suddenly off throttle, no load, for a shift.
In an automatic, the engine isn't as likely to see high RPMs. And when it does, the torque converter and shift solenoids drop the RPMs while keeping the load on the engine fairly steady.