Heals have a 40 yd range. CC has a 30 yard range ( Poly/Fear ). If the melee are pushing in, then the healers will get dragged forward. dropping a cast on a healer and blinking/blazing speeding out isn't that complicated. Locks can shadowfury heals at range with an instant cast, etc. My point is, putting any bit of pressure on a team's healers when they are pushing is huge. Even a BM throwing a single barrel onto the enemy healers can cause a problem. If the melee dps overextend, they die. If they don't overextend, and stay within range of their healers, you just alleviated a ton of offensive pressure.
Also, CC counters melee. Sheep a dps, it can't hurt you. Sheep a healer, it can't keep it's dps up. Don't tell me that CC can't counter a melee offensive push