This IpAddress is described simply as the address of the IKE server, i.e. the local address this virtual router's IKE service listens on. An admin sets this to a specific address (rather than any/all interfaces) when a device has multiple IP addresses and IKE negotiations must be pinned to one of them. For example, an admin running IKE on a specific management or public-facing address would set this object to that exact address rather than leaving IKE bound system-wide.