Optimization provides a number of different benefits, and I would stress the two most important ones.
One of them is the ability to process very large amounts of various data quickly. The number of various possible operational plans could easily be in the order of billions.
The best one can hope to evaluate and compare manually or with tools like excel is maybe a dozen of them and very little hope of finding the best one.
Add to that an evaluation of different operating strategies and things can get out of hand very quickly.

The other is the direct savings that are attributed to the ability to select the best operational plan, and the visibility into the costs of various decision trade-off. In some way, OR and optimization help to keep the organization honest, even if the desicion is not to follow the tool’s recommendations, they do not have to guess what the cost of the decision is.