Why would a network manager benefit from having network management tools?
Because it is easy to control and find the problem; when you find the problem, with having network management tools, you can easily troubleshoot and verify the problem with replacing component with a new component.
There are some scenarios; a component of the network fails or one of the component of the network is about to fail or is not working properly. Component is attacking in the network. Alert packet drop of loss, because of levels of traffic exceeds threshold of a link. Or the best scenario, the network is working without any problems.