It is not unusual to have several projects use the same set of resources. When this is the case, it's cumbersome to manage the same resources in several different project files. You can't easily see what each resource is doing for all projects. You might want to have Microsoft Project store the resource information in one file and only the assignment information in the project files. You do this by entering all resources in one project file (which might not even have any tasks) and by instructing the other project files to use the resources defined in the file with the resources.
If a number of projects share the same list of resources, you can open all the projects at the same time and view the allocation of resources across the projects. Microsoft Project warns you when a resource is overallocated because of conflicting assignments in different projects, and you can use the leveling command to resolve the resource overallocation by delaying tasks in different projects.