Active Directory has some serious replication capabilities built into it. When adding a new user or group into AD, you certainly want that change to quickly flow to all of your Domain Controller servers, so that the new users can successfully log in to a computer from anywhere in the domain. The automated spreading of this directory information is kind of the whole point of running AD in the first place, right?
The same is true for Group Policy information, as you already know. Create a new GPO on DC1, and within minutes (usually) that new GPO shows up on DC2 and DC3. What if another administrator made some changes to an existing GPO on DC2 at the same time? No worries; replication is fully prepared to handle the synchronization of settings in all directions and those changes will show up successfully on DC1 and DC3, even while the new GPO changes from DC1...