The net value of doing additional mining is that the payment that you accept as a merchant is that much harder to undo by an attacker.
If you accept that a bank vault that has one guard is made more secure by adding another guard you must accept that adding miners to a PoW chain makes it more robust against attackers -- this is the value.
If you accept that a bank vault that has one guard is made more secure by adding another guard you must accept that adding miners to a PoW chain makes it more robust against attackers -- this is the value.