The ability to assign levels-of-detail (LODs) for our meshes is a basic feature in any real-time rendering engine. It allows for MUCH more freedom in how much detail we can add to our environments with regard to performance cost. This is crucial for creating very large complex scenes, like an urban city, or a dense forest, with lots of small details that are only meant to be visible upclose.
Another feature to consider, is LOD fading.Newer game engines support this feature, where instead of the model jarringly switching between LOD levels in too noticeable a way (otherwise known as "LOD pop-in"), the LOD "fades" (like a gradient) into the next one. LOD fading isn't an absolutely essential feature, but it does look much nicer so it would be nice to have it at least as an option if LOD systems get added.