Flatlock seams are not restricted to Adidas apparel, flatlock seams are a stitching method that provides a strong and flexible seam. Flatlock seams create more holes, which means sweat can more easily exit the clothing. Flatlock seams are usually combined with Climalite fabrics in Adidas products: Climalite 'wicks' sweat out of the clothing, keeping people dry in sweaty sports pursuits and for warm climates; flatlock seams are a logical companion.
The above image - from an Adidas sweatshirt - shows a smooth flatlock seam; some flatlock seam can be bulky if it's an overlock seam. The other advantage to smooth flatlock seams is the protection they provide against chafing and the possibility of the seam irritating the skin. Adidas Climalite clothing is lightweight and tends to be skin-tight: which means that flatlock seams are essential to protect against chafing.