Lets say that my b is:
b = 2*a

but to determine that b = 2*a and not b = 3*a or some other coefficient I had to look up giant table. After that I define some loss function and optimize a  with Adam. After a is updated I want to find new value of b without doing look up again.
You can argue that I just need to save my  coefficient in some variable and use that, but my calculation is really complex and I would spend a lot of time to figure out how to rewrite everything in “tensor” form.
Edit: I am hoping that b = 2*a is already saved somewhere in the graph so I can just “evaluate” it again.