Can Server Manager be used in place of Active Directory Users and Computers/Domain Management?
Why or why not?
What improvements does Server Manager offer in lieu of replacing?
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I assume you mean the program Server Manager that is enabled by default in Windows Server environments comparing to AD services, which is something completely different on its own.
Server manager is Windows Server's interface that allows you to manage any and all roles/features in the Windows Server environment including adding new roles/features as well as changing their settings or removing them.
Active Directory (AD), is one of these roles a Windows server can perform that allows you at the very least basic to well advanced domain management including, but not limited to, creating and managing user accounts, among other features.
I strongly suggest that you read further into both before doing anything else to the server you are working on.
These links will help you get started in learning about these services.