I love the Wonderbolts-as-a-military-organization fanon that pops up every so often. This is a less boring variant of their boring uniform to suit that purpose.
Why Rainbow Dash?
BECAUSE WHY DON'T YOU LIKE RAINBOW DASH.
Don't ask me how she uses the gun or the grenade. Or how she ties her laces. That would be like asking why ponies would make door knobs and benches with back rests.