What is the purpose of a TLOC extension interface?

Providing access to business internet is a general SD-WAN transport function, not the specific purpose of the TLOC extension feature.

TLOC extension is configured on a LAN-side interface to logically extend a neighbor's TLOC (and thus WAN transport access) to a device that does not have its own direct WAN transport connection. The device using the TLOC extension shares the neighboring WAN Edge router's transport, appearing in the overlay as if it had connectivity to that transport.

TLOC extension is not an alternate or redundant path to a transport the device is already connected to; it provides access to a transport the device cannot reach on its own.

Configuration synchronization between neighboring WAN Edge routers is handled by OMP and vManage, not by the TLOC extension interface.