It increases the complexity of the attack. Usually, stack cookies make ROP harder these days but guessing the cookie only has a complexity of 8*256 (on OpenBSD), but xor'ing the return address with another value does increase the complexity even more. And that might be good news for programs that use fork a lot (like nginx) and hence don't get a refresh for ASLR/stack cookies for every request (like e.g. sshd on OpenBSD does [which does does fork/exec to ensure ASLR/cookies are refreshed]).