I use them both all the time. Lets say I have a small antenna that I want to measure the impedance of. I cal the vna normally (on my machine I just do an open/short "response" cal). But after this cal the reference plane is at some connector plane, not necessarily at the exact antenna feed point. I then take the antenna and solder on a short wire between the feed point and the ground plane. Hook the antenna up, and sweep the frequency (say from 902 to 928 MHz). As I look at a polar display I SHOULD see a small reflection coefficient dot at mag=1, ang=-180 deg.
What I usually see instead is a squigly worm somewhere else. I go to the electrical delay, and change the value until the squigly worm display looks the most like a single dot (it will never look exactly like a dot, just make it as small a radius of squigles as you can). THEN you go to the phase offset and turn it until the center of the "dot" is right on the -180 deg angle.
THen you unsolder the jumper wire to ground, and can measure the antenna's true impedance.
It is a little crude, but for under 6 ghz it works fairly well.