It depends, really. I'm not sure we're talking about the same thing, but... here's what I promised, cooked up some sloppy, fast examples...
This is a mesh light illuminated scene (ignore the porn). The computer screen acts like a screen in real-life, emitting light, and illuminating the keyboard.
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As you can easily notice, different images applied to the emission option of the screen's surface produce different light hues and intensities. That's an example on how mesh lights can be visible in a scene as actual part of the scenery.
Now, a more basic example. A single cube.
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very low intensity, you can see something's there, but not exactly sure what it is... crank up the emission strength, and you have... this.
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You can't see the cube much, it practically blends within the backdrop on this fast, sloppy render. Now, let's move the cube out of the view angle, raise it above the canvas borders...
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it's getting dark again, let's add some traditional light... a single distant light, on default diretion, and default settings... we get this...
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Let's add the default skydome to it, shall we?
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now, let's remove the distant light, and leave just the illuminating cube and the skydome...
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As it's easy to see, emissive surfaces, or mesh lights, can work fine on their own, or complimented with a skydome and/or traditional lights. And, the ways to achieve results with them are limitless, you can use them on screens, from large TV sets to smartphones, to bed lights, to even blinking led glow, as long as it helps add light, at the right dosage, to your screen. Below, it's a small, lazy render of mine, of a girl masturbating while watching porn, using just the monitor as a light source, and nothing else at all. Post-render editing in photoshop, to boost some details up, but that's about all the tampering done
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