What if someone looks up an internal domain which is actually publicly exposed. Granted they shouldn’t be secret, doesn’t mean you should broadcast them publicly.
Internal domains generally resolve to private IP ranges, so don't do any harm when broadcasted. Besides, many people use Chrome, and who knows what they do by default with browsing history.
Information leakage of any kind potentially gives an attacker something they can use. On its own any given piece of info may seem harmless, but when combined with other info it can start to pose a risk.
Leaking internal hostnames could allow an attacker to build a view of an internal network ahead of time, before actual penetration. Once inside a network it's often a race against time, so this can make a difference.
