Well if you are the kind of person that needs a rain gauge to know it's raining, I am not surprised you don't understand what I was trying to say
C'mon, you know what I meant. Humans had to calibrate the thing. Humans had to know such a thing was possible. Someone just thought it would be handy to have a device that could be relied on as a sanity check and for those without the experience to know how to tell light levels. It's a thermometer for light. I can give you a rough guess at the temperature outside, but a thermometer can tell you on a scale. It's more precise and consistent, but that doesn't mean *I'm* not accurate. I usually defer to the thermometer, but I can also tell when a thermometer is reading high or low. The same with light meters.
The main advantage of a light meter is precision. I can say that the light here right now is roughly PanF at 1/60 at f/11 in the sun. I would not be stunned if a meter suggested f/8 but I'd question f/16. I can say for sure that 1/500 at f/16 would not be enough, and 1/2 second exposure at f/1.5 would be way over-exposed. The more experience I get, the more precise I get. It's just using common sense and experience. A light meter can be a learning tool substituting for film as well as a sanity check to make sure you aren't just being dumb.
A light meter is not mandatory and doesn't improve the image at all. You still have to expose for your subject. If the subject is across the street in the shade, you need to know that you should open up the lens a bit compared to the light you are standing in. And you should know what the meter says at noon where you live. Here in the winter the sun never gets very high, so winter noon is like later afternoon in the summer. Use that as a starting point, and you can feel pretty comfortable shooting outdoors during the day. Expand from there.
I prefer a spot meter, but it still isn't sufficient by itself. You need to learn how to use it and how to interpret what it says before it "does the work for you." And I'd suggest that learning to effectively use an in-camera spot meter is no harder than predicting what it's going to say.