New Geomancer Player Using “Dia”

I went with a few groups and players asked me (Geo) to use “Dia / Diana” after using indi or Geo spell.
What is the main purpose? Won’t I overwrite higher tiers Dia spells if applicable?
Thanks

We need more context here, what are you doing?

If youre fighting back to back mobs with the same debuff geo/indi, you need to get on the hate list for the debuffs to affect them. This doesnt have to be dia either. You can cure someone with enmity already.

If youre fighting a one-and-done target, you dont need to dia the target. Casting the bubble on that target puts you on their hate list.

If you're dropping an offensive bubble (like Frailty or Malaise) and they pull continuous mobs, the new ones won't be affected by Geomancy until you get on the monster's hate list, as Nynja mentioned. The party is telling you to make the bubble active for every new mob that is pulled, that's all. You can accomplish the exact same thing by curing a party member who has acted on the new monster, or paralyze the mob, dispelga, anything. Just need to get hate on the monster for the bubble to affect them. If you're using a bubble that buffs the party, that hate-action is not necessary, since you get the buff always.

Casting Dia of a lower tier (Diaga included) does not overwrite a higher tier.

there is a neat trick to it also once any 1 engage u can barspell aslong as the buff reach them. When i used to join MLVL parties with JP players that was the strategy each dd pulls 1 4 dd's 1 rdm 1 Geo so tag all lol, was like 400k an hour quite insane after aoe nerf