In situations where conventional light shapes will not suffice, mesh lights are more suitable. Mesh lights can be used to create interesting lighting effects that would not be possible any other way. For example, effects such as neon lighting or a car light motion trail effect can be achieved more easily with mesh lights.
The example below is a comparison between a mesh light and a surface that has a standard_surface shader assigned to it with a high Emission value.
You can see that even with diffuse samples = 16, the emission is noisier than a mesh_light with diffuse samples = 2.
Below is another comparison test between a mesh light (left image), and a sphere with a highly emissive standard_surface shader assigned to it (right image). As you can see in the mesh light image, light is bounced around the scene, whereas there is no bounced light in the emissive sphere render. This is because the rays from the mesh light work in conjunction with the Diffuse rays, whereas, only the primary rays are considered when using emission only. Even with 6 Diffuse samples, the render using Emission only contains far more noise.
In the example below, Diffuse samples have been increased to ten to get a clean result using only Emission in the scene. With mesh light, the scene only requires three Diffuse samples.
Connect a polygon mesh to this parameter to create a mesh light.