Without this, a gift box is either an untracked product with no cost, or you manually adjust several items every time you build one. Both produce wrong margins — and gift sets are usually made in the weeks when getting margins right matters most.
The same mechanism covers the opposite case: buying a case and splitting it into units to sell, or breaking a bundle back down when the season ends. Recording it as a dismantle keeps the component stock honest instead of leaving a phantom set on the books.
Worth deciding up front: whether the set is a permanent product you rebuild regularly, or a one-off for a promotion. The first is worth setting up properly; the second is often simpler to handle as a discount on the components.