Flat HP Seals vs Scaling HP Seals. Which is better?

So Ive been buying scaling hp seals because they're half the price of the flat hp seals and flat hp gives you 8 hp (x9 is 72 hp extra at level 1), but with scaling hp seals you'll have the same hp value at level 6 then you would with flat hp. This sounds really confusing, so heres it in simple terms: Level 1 base hp with flat seals = x+72 Level 6 base hp with scaling seals = x+72

So my questions I guess are

Why do supports often prefer the flat seals over scaling? I understand it makes your all ins before 6 very slightly safer but it seems like having an extra 216 hp at level 18 would be better than having 72 extra from 1-18. Are they really worth the extra 400 ip each for that level 1-5 (when odds are you won't even be able to all in before 6 in a lot of matches)

I don't play support too often but its kinda fun sometimes and I want a proper rune page setup if I end up playing support in ranked. (still need to buy the tier 3 armour quints)

Quick edit because of the first 2 commenters both using a vs. Leona/Thresh situation as to why flat is better.

The main Supports I play are Thresh and Leona. Both good early engages and not exactly squishy. I don't even engage early if my adc isn't farming well because it makes me not trust them as a player lol. I just wanted to see if there was some real important reason why flat was better.