Why is specificity between a receptor protein and a signal molecule important?
Lets take an imaginary receptor, signal molecule and response.
Lets say that activation of this receptor induces a cell to destroy itself. This is something that should be avoided unless absolutely necessary.
This receptor is activated by a signal molecule called X. X is released when a nearby cell gets infected with a bad disease and death of the surrounding cells is needed to contain this disease.
X is released and the surrounding cells die. All is relatively well now.
But what if this receptor was not so specific? What if it could work with more than X?
When this receptor would not be so specific as to only work with X, a lot of these cell death reactions could occur, leading to the death of the organism.
This specifity is needed to make sure that processes only happen when they are supposed to happen