Aerial forces are often decisive in combat situations. Conventional historical battles before the 20th century essentially had no aerial forces (other than balloon observation posts, used for reconnaissance rather than weapons platforms). So the addition of fantastic aerial forces, either through use of magic, anachronistic technology, or fantastic creatures, can make a huge difference. Mass battles in many game settings will plausibly more resemble modern warfare, with diverse forces with different capabilities on land, sea, and air, rather than simply a few thousand men charging at each other across a muddy field.

The other consequence of this is that traditional medieval style defences, such as plate armour and castle fortifications, become much less useful. There's a reason people don't build castles for defence any more, and soldiers don't wear plate armour or chain mail any more. Modern attack forms render them much less effective, and aerial dragon squadrons and flying wizards would do the same.

Just something to think about when running a fantasy game.

Transcript

GM: Oh, all right. I guess you can join in.
GM: This is a significant battle between massed NPC forces. We can apply the mass combat system with you as an Ewoc commander, Sally.
C-3PO: Right. What aerial forces do we have?
GM: Um...
C-3PO: They live in treetop villages. They'll have gliders.
R2-D2: Attack gliders.
C-3PO: They drop rocks on the troopers.
R2-D2: Rocks?
C-3PO: Attack rocks on wings of economic opportunity! Fly my pretties! Swift as the market trades!
Ewoc pilot: Assets away!!
[SFX]: Krunch!
[SFX]: Kranch!
C-3PO: Trickle down theory at its finest!