When am I supposed to use flare?

Been leveling my black mage job, level 55 now, and I can't figure out when I am supposed to use this.

It completely drains your MP and switches you to aetheric fire (Why?? I have no MP). I'm probably just unaware but I don't know when I should be using this for AoE over Fire 2. Any ideas?

Convert and Transpose are your friends.
Usually it's done at the end of a aoe rotation with swiftcast and transpose up, so you fire II till you've got just enough mana left for Flare, swift cast>flare>transpose>wait for tick and on your merry dps way.

I like to use it as a finisher aoe before i transpose into umbral. You can double cast it with convert/swiftcast or even triple cast with an x ether until lv 60.

Well, first, Flare is one of the most powerfull aoe of the whole game, even with the nerf it received with Heavensward. This reason alone should make you use it as often as possible.
Now for the MP cost, well, use it when you are almost done with your Fire 2 spamming,
Like Fire 2->till almost no MP (just enough for Flare), Flare, you have now 0 MP left, use your skill to switch to Umbral Ice, Ice 3, get back your MP, Fire 3, repeat.

Now you can also use the classic Twin Flare method with
Fire 2 till few MP, Flare, Conversion, Swiftcast, Flare, switch to Ice. Huge damage in short time !

Non-level 60:

Single target: only flare when you have convert and swiftcast up, usually in your opener and when RS is off cd.

2 targets: Fire 1 spam then finish off w/ a flare > transpose > F3 > repeat. If convert is up then flare > convert > flare.

3+ targets: Fire 2 spam then flare > transpose > F3 > repeat.

Level 60:

Only flare on 3+ targets or if you are using the sharp F1 opener and don't get a F3 proc on your second F1.