[SR-Users] Why append_branch should be called in a failure route ?

[Vitaliy Aleksandrov]

Below is the example from http://kamailio.org/docs/modules/3.2.x/modules/tm.htm :

failure_route[0]{
	if (t_check_status("5[0-9][0-9]")){
		# I do not like the 5xx responses,
		# so I give another chance to "foobar.com",
		# and I drop all the replies to make sure that
		# they are not forwarded to the caller.
		t_drop_replies();
		
		rewritehostport("foobar.com");
		append_branch();
		t_relay();
	}
}

Why do append_branch() is present before t_relay().
I'm asking because I didn't used it in failure routes and call rerouting on various conditions worked great.