PowerShell has become a core component of Windows and an essential tool for system administrators. If there is one word that sums up the reason why, it is automation, for which GUI tools are unsuitable. Anything that can be scripted can also be automated, and automation is required for DevOps and for administration of cloud platforms. Another factor is that Windows Server does not always have a GUI. Server Core and Nano Server are stripped down, making PowerShell essential. If you use Office 365, services such as Exchange Online can only be managed via a web browser, or through PowerShell, and it is PowerShell that provides the most complete and powerful access. Even if you run Exchange on premises, PowerShell is required for some tasks, and it pays to know the PowerShell way of doing things.
PowerShell is particularly valuable for tasks that involve iterating through lists. Active Directory is an obvious example. The Active Directory module is installed automatically on domain controllers, or when you install the Remote Server Administration Tools. The Get-ADUser cmdlet retrieves all users into a collection. You can filter the list or use a foreach loop to inspect each user object, and use Set-ADUser to modify user attributes or group memberships.