despite their different mathematical nature, what is the visual difference?
Probably this helps you:
Add
The Add expression takes two inputs, adds them together and outputs the result. This addition operation is performed on a per channel basis, meaning that the inputs’ R channels get added, G channels get added, B channels get added, etc. Both inputs must have the same number of channels unless one of them is a single Constant value. Constants can be added to a vector with any number of inputs.
Multiply
The Multiply expression takes two inputs, multiplies them together, and outputs the result. Similar to Photoshop’s multiply layer blend. The multiplication happens per channel, meaning that the R channel of the first is multiplied by the second; the G channel of the first is multiplied by the second, and so on. Both inputs must have the same number of values unless one of the values is a single float value.
I got those informations from here: https://docs.unrealengine/latest/INT/Engine/Rendering/Materials/ExpressionReference/Math/index.html#multiply Just take a look at the pictures, then you will understand it 
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@ i think you missed the point of this thread.
://cgtextures/content.php?action=tutorial&name=blendmodes
good tutor on photoshop blendmodes that work similar to these nodes, still very different but you get some idea
Wow, great article, thanks for sharing!
Short answer:
“Add” lightens
“Multiply” darkens
Cheers,
Michael
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