left-child, right-sibling is a way to convert any (multiway) tree into a binary tree by “rotating” its branches.
This representation is more space-efficient, but comes at the cost that looking up a node’s children by index is slower. 1
Another benefit is that since each node is now binary, we can store exactly 2 pointers (or indices) per node rather than a dynamically allocated structure like a dynamic array. This is useful when storing nodes in a flat array.