In some scenarios in which you have a hub router that connects to multiple paths to a specific site, only the hub router is aware of the redundant paths to the same destination. When it advertises routes for that destination to other neighbors it only advertises the single best path to that site.
Add Path support allows for the hub router to advertise all possible paths to the same destination so other neighbors can send traffic to it on both links, or have the redundant path in the routing table for immediate failover. The only requisite needed by the hub router is to turn off split horizon on the multipoint tunnel interfaces toward individual spokes.
The Variance and Add-Path features are not compatible with each other. when using the Add-Path set the variance to 1.
no-ecmp-mode is recommended if the hub router uses multiple tunnel interfaces to reach the spoke sites.