It doesn’t though?
It lasts 15(21 with hero talents) seconds and heals nothing and it’s on the GCD.
Unless you can heal for more than a heal extra from the renew being on someone, it’s not worth the GCD, you might as well just cast a heal like serenity, flash heal or heal on them than spending the GCD on renew.
Let’s show how it goes:
With 5 casts, casting renew: renew, 2x flash heals(636k each), 2x heals(848k) each. This will produce about 3 million(3.2 million when you include the hot which takes 21 seconds to heal all of it) healing or something, from the 5 casts.
With 5 casts, not casting renew: 2x flash heals(600k each), 2x heals(800k each), 1x flash heal(600k). This will produce about 3.4 million healing from the 5 casts.
Let’s check 6 casts with renew: renew, 2x flash heals(636k each), 2x heals(848k), 1x flash heal(636k). Total healing: About 3.6 million(and then some 200k from the renew over 21 seconds), barely beating 5 casts without renew.
6 casts without renew: 2x flash heals(600k each), 2x heals(800k each), 2x flash heal(600k). Total healing: About 4 million healing.
7 casts with renew: renew, 2x flash heals(636k each), 2x heals(848k each), 2x flash heals(636k each): Total healing: About 4.2 million healing(and then some 200k from renew)
7 casts without renew: 2x flash heals(600k each), 2x heals(800k each), 2x flash heals(600k each), 1x heal(800k). Total healing: About 4.8 million healing.
8 casts with renew: About 5 million healing(+200k from the hot from renew).
8 casts without renew: About 5.6 million healing.
Why would you press renew? You heal less if you press renew.
This is without even including mastery btw, renew doesn’t activate our mastery. With mastery included it skyrockets well beyond what pressing renew will do because you’ve cast another heal that activates the mastery instead of spending the GCD on something that doesn’t even activate it and even if it did activate it, it wouldn’t trigger a strong enough echo of light from the little healing it does.