It's similar to timing Earthy Star to blow after a raidwide to heal it. Let's say a single raidwide is coming up in 5-10 sec, your party is healthy and the next raidwide after that one isn't for another 10-30 sec. Extremely common scenario. Raidwides rarely hit back to back unless it's a Tumult type (which Lily also handles).

-Star, you'd set it 5-10 sec before the raidwide. The raidwide hits. Let it reach Earthly Dominance. Pop it or allow it to pop (+720 potency). Your party is healed and safe for the next raidwide.

-Lily you'd set it 5-10 sec before the raidwide. The raidwide hits (+400 potency). Let it expire. Lily automatically uses 4 excess stacks (+800 potency). Your party is healed and safe for the next raidwide.

It's a full Star and a half worth in this scenario. Why would you need a GCD heal and how is that massive backloaded potency useless? It only has a 15 sec duration, it'll pop well before you're in any danger. Raidwides don't need healing instantly.