I feel like I see and use this a lot and never really thought about it until now. I understand that the multiply node multiplys the A pin by the B pin but what if there is nothing connected to the B pin, how does this change the value of the A pin and what exactly is it multiplying now?
Well, it’s kind of simple really 
A multiply node exists of 3 pins, 'till you add more ofcourse.
There’s pin A, which has a value of 0, there’s a pin B with a value of 0 and an output pin.
So, if you connect something to pin A and nothing to pin B it will result in the following
- 0 = 0
In other words, if you use multiply and leave one of the pins empty, the outcome will always be 0.
I hope this answers your question, don’t forget to mark as answered if it does
It uses whatever value is set inside the node itself, which by default is 0.0 for a (float * float) Multiply node:
Note that this default value is different for the Multiply node in the Material editor - which defaults to 1.0:
