Why are some mix nodes set to Mix and others to Multiply? What is the function of each? What instance would you use the Mix and visa versa?
good question!
In short, Multiply darkens/influences the image input, where Mix allows you to set a new value.
If I Multiply the puddle matte into the brick texture, the resulting texture will be the brick texture but darker in the puddle areas. This is fine for the color and the roughness, because I want the brick texture to still be there. When it comes to the bump though, I want clear unbroken puddles that can reflect the sky. If I had used Multiply, the result would still be influenced by the brick texture, and the puddles would be bumpy. For my example, I Mixed to a new flat color, to simulate the flat surface of the water.
Try it out yourself to see how it works! there's no right solution to this... for your scene, you may not want clear puddles, but you might just want damp bricks. In that case, setting it to multiply and adjusting it might be the better solution!
If you are interested in this, you should definitely research Blending modes... much better people than me have written much better explanations than I can here, haha.
