Merge-descriptors efficiently copies properties (including descriptors) from one object to another. Unlike simple object assignment, it preserves property attributes like enumerability, writability, and configurability. This is crucial when crafting APIs where you need fine-grained control over object properties. Typically used for inheritance or extending object functionality without shallowly copying values, ensuring a more robust and predictable object structure. It's a focused utility for advanced object manipulation in JavaScript.