One of the consistent struggles of working in a “standalone” project management environment is the fact that you do not have visibility of the “total picture” associated with resources. Resource loading in MS Project provides visibility of resource utilization vs. capacity on your project – not across all resource commitments (e.g., other projects and operational activities). You can utilize the “max units %” to reflect that a resource is something less than 100% available to the project, but this information is only relevant if it is regularly reviewed and updated. In my opinion, the most significant benefit associated with Enterprise Project Management tools such as MS Project Server, is providing enterprise wide visibility of planned resource utilization vs. capacity. As a by-product of maintaining resource loaded project and operational schedules within MS Project Server, up-to-date resource utilization information is available in a very flexible and easy to access and consume manner

Read more here:
Using MS Project Server for Resource Management