What is it?
An essential shader node for projects with cities or roads. Not just traffic signs and safety equipment, but almost all cars, have retroreflectors. They reflect light back to the source, this doesn't happen with regular materials. The hexagon patterns or stripes they have can be generated with the shader too.
Why do you need it?
Easy to use
Get the result you want within a few clicks. The default values are often already good and the settings are self-explanatory. Because of this it can be faster to set up than a regular principled shader. You won't need to endlessly fiddle with sliders until it looks the right kind of reflective, which isn't even possible with normal shaders.
Retroreflections are not completely visible in Eevee yet when the light source is off-screen, so at this point in time using cycles gives the most accurate result.
It takes your realism to the next level
If there's a road visible, there's reflectors. Yet almost no CGI renders you can find online use it. Try it! search for "blender street at night" on google images, they all have regular materials for stripes on the road, no proper reflectivity.
If you use the reflector shader in your render, people instantly notice it as something that normally isn't seen in computer graphics. It sets your work apart from others, taking it one more step away from looking like CGI and closer to photorealism.
It's versatile and works with any texture
This means that you can use it on the assets like traffic signs you already have, by simply replacing the existing shader and using the same maps for colour, roughness, normal etc.
The texture used below is just a black and yellow arrow. The shader itself can then create detailed patterns and reflections when lit.