With ExpressRoute Global Reach, customers are able to link ExpressRoute circuits together to make a private backbone network between your on-premises networks via the Microsoft global backbone. With that, Microsoft becomes the global backbone provider of the customer.
That enables customers to always choose the best local MPLS or WAN provider in their area, link them to an ExpressRoute and afterwards link the ExpressRoutes together to build a global Backbone Network.
The following diagram shows a schematic overview: