If you watch me work, one thing you'll notice is that I use a hand-held light meter. This has been part of how I shoot going back decades, and for a good reason -- excellent images require proper lighting, and proper exposure that balances that lighting. (Yes, you can do a lot in Photoshop. Adjustments are normal, but fixing poor lighting and ratios after the fact as SOP marks an amateur; pros only do it when circumstances force our hand.)
After years of using light meters, here's what I've learned:
1. Light meters are stupid. They have absolutely no idea what the correct exposure is.
2. Light meters in the camera are even stupider and have even less idea what the correct exposure is.
3. But, I hate working without them.
When I was shooting all three of these images, the correct exposure was very different from what my light meter said. Based on it, the first image is over exposed (high key), the second is under exposed (light supplemented) and the third is over in some places and under in others (back lit with fully blocked foreground shadows).
Actually, all three are correctly exposed. Correct exposure is not an objective value. It is an artistic value based on what you're trying to accomplish in the image. So why use a light meter? Because used correctly, a light meter gives you something useful: the exposure that yields neutral gray in a given light. Unless I can't for some reason, I measure the light falling on the subject (incident metering) rather than bouncing off (reflected metering) because colors and tones alter reflected light (meters in cameras always read reflected light, btw), making it harder to determine the objective, neutral gray value. It's my job to decide what tonal value I want the final image to show, not the meter's. The meter gets me there faster by giving me a known start point. Once I know the tonal values in a setting, I set my camera (on manual) to the exposure I decide is correct, and I'm usually there with only a couple test shots and adjustments.
Yes, I know your iPhone gives you good exposures. Camera designers have gotten really good at creating light meter systems that give snapshooters good automated results in average situations most of the time. But, even with advances in the technology, these systems still struggle in many lighting conditions that, ironically, are some of the best conditions for dramatic shots. Many strong shooters use auto exposure, but no serious photographer blindly trusts automation without paying attention to what it's doing and overriding it when appropriate.
So, of course light meters are stupid. They have no brain and they have no heart. But, they're fabulous tools that your brain and heart can use when you shoot.