SDWAN and firewall positioning

Hello, I read a silverpeak article whichs recommends to put the SDWAN device inbetween 2 virtual routers on a palo alto firewall essentially making the PAN device the externally facing device. Is there a reason I wouldn't just put the silverpeak on the edge? Is it not secure enough to be an edge device?

https://www.silver-peak.com/sites/default/files/UserDocuments/PAN-Deploy-TopNav/content/edgeconnect_topics/service_chaining_to_an_edgeconnect_branch.htm