The purpose of a heat trap is to prevent heat from rising out of the water heater by convection and into the piping, there to radiate into the surrounding air. For years, we’ve advocated using an inverted U of copper or stainless steel flex lines for this, and insulating the flex line to the top of the rise. Heat only rises. It doesn’t fall.
Our passive heat traps are illustrated in the photo at top right.
In the early 2000s, the government started mandating heat-trap nipples, to do the same thing. The first ones used a little marble, while later ones used flaps. In practice, they have occasionally caused problems, such as the ball rattling, or the devices malfunctioning and shutting off water flow out of the heater. They also constrict water flow.
The pink and blue inserts in the nipples at bottom right are heat traps.