Understanding Wireguard selective routing tutorial

[gstyle]

Hi,
I set up selective routing following this tutorial:
https://docs.opnsense.org/manual/how-tos/wireguard-selective-routing.html

Works fine.

However, I do not really understand "Step 9".
Outgoing traffic from the VPN address, that does not go into the VPN net shall go through the VPN gateway.

What is happening here???

[hushcoden]

I'd like to understand what that step 9 is for - I have wireguard working without implementing that  ::)