The Magpul RSA adapter functions as a sling loop with a QD clasp (specifically quick-detach), which is primarily designed for mounting on the handguard and attaches to the universal Picatinny rail. Thus, it can serve more than just for mounting additional grips or optical sights.
The Magpul sling loop is designed to be as unobtrusive as possible when handling the weapon, primarily thanks to its low profile. Additionally, it is extra strong, as it is made of carburized steel, which ensures great resistance to weather conditions.