What happens if someone steals one of my additional domain controllers? What information could an attacker get from my domain controller?
Your domain controller needs to be able to authenticate users. So it needs to be able to verify user/password combinations. Thus it needs to store password information.
This is not stored in plain text, but worst case people will eventually crack those.
Having an DC get stolen is therefor a bad thing.
The DC will also contains lots of other information, such as computer IDs, security groups, membership to security groups, DL list, ... All things which you might not intend to share but not as bad as the [domain admin] passwords.