Skin heals better
Has an additional sleeve which protects the sharpness, not like others that bend easily, which can cause bleeding or scarring or problems. In between the needles is space to hold pigment, for better deposit of color. Flat has a finer point than round, better for eyeliner and eye area because there is less trauma to the skin. Skin heals better and locks in more color as a result. More color means less fading!
Flat needles are used around the eye because it delivers a straight line rather than zig-zag and heals much faster.
Flat needles can also be used for outline, brow feathering, brush hair strokes, alignment, or fill-in lip shading at a 45º angle. Lips are a mucous membrane and have a much different texture than skin.