This is a general knowledge question I can't wrap my head around. I've set up many of IPSec tunnels throughout the years both for site-to-site and to direct internet traffic through a tunnel.
I just learned of "route based IPSec" and I've always done it policy based where you set the left and right subnets. These subnets explicitly appear in the IPSec SA.
The more I read about IPSec I've seen blog posts about IPSec over GRE and GRE over IPSec. What is the added benefit of the overhead of an additional header? I've always just routed traffic through tunnels without this and everything was fine