Think of sitting or standing in a slouched position, and someone tells you to sit up straight and get good posture. You use your back and stomach muscles to straighten your spine and elongate your body.
You do the same when you're swimming. Try swimming without using your upper body at all except your arms. Your body will swing around a lot. As you stretch out and tension your upper body, you'll find you flow through the water better. If you do the equivalent of standing tall, you'll find you can also get better reach.
Also, when you're rolling - your shoulders want to rotate along with your hips. You need tension in your upper body for your shoulder and hips to rotate together.
Tension isn't a great term because you should still feel relaxed - but if you think about standing, you're using your muscles all the time to stand up straight, but that doesn't mean you feel tense - there are core muscles that you use all the time for balance, without feeling tense. Hope this helps.