It demonstrates that the client actually read the contract.
Discounts after the contract is approved are actually a headache in some large orgs. You will have to answer multiple questions each quarter as to why your numbers are off. You won't get to spend the money elsewhere, but will have more work. Plus, you'll never get budget approval for buying or shipping a plant.
Plus, if the budget has already been approved, someone didn't read the contract before signing it. That's the same as not reading it.