It frees you from expensive complexities arising from tunneling and private address space. You don't need the network engineering and service costs to bring your private 10.x overlay network to your various datacenters[1] behind different ISPs, you don't have to fight with addressing conflicts when talking to other people using rfc1918 space or debug situations where 1918 addresses are ambiguous, etc. It's cheaper, safer and simpler to integrate with other organizations because you only have to configure your firewall rule instead of trying to mash together incompatible rfc1918 internal networks and overlay technologies in addition to the fw. Your security posture is better and cheaper to maintain because your simpler network is easier to reason about.

[1] meaning your network locations, not your private datacenter (necessarily)