Since we're talking about programs that are trying to set up a connection no matter what, I'm going to say "not much". It's not significantly shrinking the attack surface and forcing attackers onto a plan B that's meaningfully harder to do. It just adds this layer of awkwardness to everything, and attackers shrug and adapt.

You block inbound to block inbound. Of course it doesn’t do anything for outbound. Acting like you can just turn inbound filtering off because of that is disingenuous.