One reason would be to not expose details about your private network to every hop the ICMP packet traverses. Even if knowing you have some 192.168.1.x host is not on its own very useful to an attacker, it'd be preferable to not expose that.
It's another reason WebRTC/STUN was a big issue when it first became widely available, it made it easy to leak details about your LAN to outside servers.
It's another reason WebRTC/STUN was a big issue when it first became widely available, it made it easy to leak details about your LAN to outside servers.