The point of Celestial Body rune?

Tanks. I take it as a tank support all the time.

i use it as a tank supp, you get 200 health from the very start, and the debuff goes at 10 mins so its not that bad

I take it on poke supports if I'm facing something that can either poke me back or all-in me reliably. That 10% damage nerf is far from a big deal if it lets me proc Spellthief more often. In fact, I used to take it on Sona until they nerfed her.

I've also seen people use it against me when playing a poke support : I remember this Jhin that took it with Grasp and a doran shield, effectively making him unkillable during laning and it didn't stop him from pulling his own during skirmishes.

Honestly, the 10% damage nerf on Celestial Body is overated, especially when the rune is meant to be taken in losing matchups anyway : It's not like you were going to be dealing constant damage on those first 10 minutes while desperately trying to farm and survive anyway.