Learning to measure light without instrument is useful also because creates "expectations" about the reading of the light meter. If the result of the light meter is far from expectation, one might realize that the ISO setting on the camera is wrong or, if the camera only shows the shutter speed in the viewfinder, one might realize that the aperture is not set to the desired value.

Also, in a "high contrast" situation (part of the frame in full sun, part in shade) the "manual correction" which one has to apply to the suggested camera reading is better evaluated if one is "trained" in estimating exposure. It's a "skill" that never goes wasted, even if one always uses a light meter.