Consider a scenario where the developer is required to write a custom directory tree. Each row contains a caret and a checkbox. When either of these are clicked, you'll need to perform some sort of tree traversal to ensure integrity of the view -- for example, checking the checkbox on a row may require that all children as selected too. Additionally, you may need an upward traversal to the root to ensure the parents are either in a mixed or unchecked state.
If the developer doesn't understand performance (or at the very least, basic tree operations like searching/sorting/traversing), they're going to write some horrible code. Google and Bloomberg have their pick, they don't want a shitty developer.
