I think only two to three ppl that have had to define a rule to route ISP to the WAN. Most ppl just put the router IP address in the Policy Routing GUI so services on the router, like NTP, can still work and get data even if the OpenVPN client goes down and one has "Block traffic if the tunnel goes down" button checked that is shown when Policy Rules is enabled.
The ISP is the backbone to the WWW and will direct traffic where you want it. It is not necessarily the endpoint.
One person had Xfinity Internet Service Provider and there was a feature that did work with the Xfinity unless they routed the ISP ASN to the WAN. This makes sense since Xfinity is the ISP and is expecting you to be coming fro the IP address they assign you.
The other reason that was provided for routing ISP ASN to the WAN is that many streaming services use a
Content Delivery Service or CDN. They cache content locally to reduce buffering and provide faster response times. For reasons unknown, the person had issues with a streaming service unless they routed the ISP to the WAN. There are many variables involved such as VPN Provider, DNS Settings on the WAN and OpenVPN Client Page, Policy Rules or RPDB priorities that come into play. This is an outlier use case that only a few ppl have reported as being necessary.
See the
Policy Rule Routing post for a basic overview of selective routing and use case examples.
).
