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 :).
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 :).