You ever tried to hipfire a heavy machinegun? Now imagine doing that in mid-air. ;)

they could try putting it on their heads

Have you ever tried to strap a gun to your head and fire it? Clearly not.

have you ever tried shooting a firearm with your mouth, which according to the wiki is a thing they do.

Yeah, never was a fan of Kkat for bringing that into the fandom, it doesn't make any sense either. At least battle-saddles won't give you a concussion.

Wait how do they use those? Are there strings they pull with their fin- mouths or something tied to the triggers?

yes

As an unnecessary addition, planes give certain advantages.

First, carry weight. Carrying a bandolier of grenades? Doable. 500 lb bomb? Less so.
Second, loiter time with heavy weights. Machines can't get tired. Flesh isn't so reliable, and even when it comes close it isn't easily replaceable.
Third, numbers. Pegasi make for natural paratroopers so why waste them on other tasks when a plane allows earth ponies (endurance, for resisting g-forces) and unicorns (which will sometimes have useful and usable magic to aid in their task).
And then you have simple speed. Sure you have (an) outliers who can go Ludicrous Speed, but endurance again crops up as it is a very rare individual who has not only trained themself to sprint for long distances, but be able to do so *at all* constantly over hundreds of miles.

Oh, and being able to armor the plane helps.

引用自 DaLintyMan：

SNIP

Wait, was this a SERIOUS question? I thought it was a joke.
Also, what he said.

no this was serious i don't get this at all lol

引用自 DaLintyMan：

As an unnecessary addition, planes give certain advantages.

First, carry weight. Carrying a bandolier of grenades? Doable. 500 lb bomb? Less so.
Second, loiter time with heavy weights. Machines can't get tired. Flesh isn't so reliable, and even when it comes close it isn't easily replaceable.
Third, numbers. Pegasi make for natural paratroopers so why waste them on other tasks when a plane allows earth ponies (endurance, for resisting g-forces) and unicorns (which will sometimes have useful and usable magic to aid in their task).
And then you have simple speed. Sure you have (an) outliers who can go Ludicrous Speed, but endurance again crops up as it is a very rare individual who has not only trained themself to sprint for long distances, but be able to do so *at all* constantly over hundreds of miles.

Oh, and being able to armor the plane helps.

Literally all of these reasons (defense, weight, and numbers) are the reasons provided for the adoption of planes in the focuses.

So wait if something like birds became sentient the thought process is that they would rather use planes instead of their own naturally given abilities? Which in retrospect would use less resources, right?

引用自 Epik Berm：

So wait if something like birds became sentient the thought process is that they would rather use planes instead of their own naturally given abilities? Which in retrospect would use less resources, right?

As someone who wrote a majority of the lore, focuses and events for many Majors in Griffonia (Griffonian Empire and Republic, among others), I can tell you that the Griffons do make use of their ability to fly in combat. However, this isn't portrayed in HoI4's "Air Combat" system. In other words, Griffons aren't "Planes" which you put in Airbases and have fight for air supremacy or bomb cities. Instead, they use their wings in direct combination to Ground Combat, thus giving them small bonuses for land battles (research the Griffon Race Techs).

The overall reason for why Planes exist at all has already been stated above. No matter if Pegasus, Changeling or Griffon, Flying is more of a civilian use method of transportation rather than combat. Or, at least nowadays it is, when everybody has machine guns, Rifles and AA Emplacements to shred you apart. Naturally, Vehicles were invented that allowed for safer airborne military operations. First lightly Armoured Biplanes, then Monoplanes which gradually received better armour and speed.

All in all, You can probably see why nobody in their right mind would grab a Rifle and fly towards an enemy position during a war with nothing protecting him but the helmet on his head. Planes are not only FASTER, they are SAFER and DEADLIER as well. Be it bombs, cannons, rockets or whatever else: You can put it on a plane. Probably.

why uses vehicles when already have legs? clear enough

well, most things have legs but don't have wings. I sort of get what you're saying but also not