Stygia
Every other layer of the Nine Hells has a function related to warfare, industry, administration, or commerce, but Stygia is an expanse of untamed, unimproved territory. Even so, it has its uses. All manner of unlikely beasts wander the frozen terrain and swim the seas, including remorhazes, krakens, mammoths, and even a few tribes of frost giants. These denizens have no fear of any other creatures, including devils, which makes the place an ideal proving ground. Lesser devils that need to sharpen their combat skills or improve their endurance before reporting to Avernus for duty in the Blood War spend time in Stygia. The cruelly cold environment, combined with the constant threat of attack, helps commanders assess their troops and place them into the various legions as appropriate for the skills they demonstrate.
The presence of so many creatures native to the Material Plane has led to speculation that Stygia was not always a layer of the Hells, but was previously a world on the Material Plane. Its inhabitants, facing annihilation, are said to have pledged their souls and their world to Asmodeus in return for a safe haven — whereupon Asmodeus kept his end of the bargain by transporting the world into the Hells. The archmage Tzunk has researched the topic extensively but has yet to find any evidence that truly confirms the account. If the supposition is true, then the riches of that world might lie under miles of ice and beneath frigid, monster-infested seas.