Hello,
I have been confused about something and I’m not sure if this is even asking it the correct way, but I’m wondering what the difference is between:
Using R,G, or B plugged into the opacity?
vs
Using an alpha channel made in photoshop?
For example, I’ve seen materials that use an alpha channel vs ones that use the RGB for an alpha. What are the different reasons behind the different approaches?
I try to explain it. Keep in mind that the information I provide is directed specifically to the opacity channel. Different channels like emissive, base color, roughness, metal etc. will behave differently, when you use alphas and RGB inputs :).
Example:
I have a diffuse map, which has a red, blue and green channel (the white pin is just the sum of the red, blue and green channel).
When you drag off the red pin for example, it will extract the red color information it finds in the texture and displays it as black/white image. This image can then be used for the opacity input of the material.
You could practically use a seperate RGB texture to control 3 different opacities, without to use three different textures with one alpha opacity. This gives you more options and can be used for layered materials, which shouldn’t have the same amount of opacity.
An alpha map gives you only one opacity option, but can be integrated into the texture itself in photoshop. This decreases the instructions of the materials and hence results in a better ingame performance.
Although you can do a lot of stuff in materials to convert textures to something different through nodes, they also increase the number of instructions on your material.
I hope I could shed some light :).
Cheers!
So would it always be better to have an alpha channel in the texture?
Only if you need it—think of a texture map as 4 channels, RGB plus Alpha, most of the time you only use RGB, but the alpha is there in case you have a grayscale image you want to save with the file.
And alternately, like was mentioned, you can put whatever grayscale images in the RGB channels instead of having those images as individual texture files.
Well a Rose by any other name. 
A mask is a mask be it called R,B,G, or Alpha and giving them a name proper saves one from going nuts if they were all called Alpha channels.
The benefit is instead of having 4 separate diffused images you can put all 4 into the single image.
Kind of like bunk beds for alpha masks.