Flatlock seems are usually used for next-to-the-skin garments such as t-shirts, long sleeve shirts, pants, underwear, gloves and hats. These garments must be extra comfortable, so they don't chafe you skin or cause any other discomfort. Insulation layers like מעילי צמר typically also have flatlock seams because the fabric is relatively thick. Regular seams are not the best option for thick garments because the seam allowance would make the garment extra bulky.
here is no doubt that flatlock seams are necessary for pants and thick garments such as fleece jackets. These garments would be very bulky without flatlock seams and the hanging fabric on the inside would limit your range of motion. However, when it comes to base layers, I can argue that flatlock seams are not always of the utmost importance – depending on the fit and fabric.
When it comes to polyester (or nylon) base layers, flatlock seams are more important.Therefore, it's more likely that the seam allowance of a regular seam will chafe your skin. Furthermore, many polyester next-to-the-skin garments these days have the so-called compression fit or tight fit which presses the seams tight against your skin.
