[Before Windows Vista], all the default Administrative Template files are added to the ADM folder of a Group Policy object (GPO) on a domain controller. The GPOs are stored in the SYSVOL folder. The SYSVOL folder is automatically replicated to other domain controllers in the same domain. A policy file uses approximately 2 megabytes (MB) of hard disk space. Because each domain controller stores a distinct version of a policy, replication traffic is increased.
Create Central Store on domain controller – Windows Server | Microsoft Docs

So looking at my environment, 10% of my Group Policies have legacy .ADM files, taking up 500GB of storage space.

Replicating across our ten Domain Controllers.

Microsoft’s alternative, was to create a Central Policy Definitions store, stored with the newer ADMX Group Policy Template files. 

It’s size, 55MB.