Threshold enzymes are used to bring key molecules in to biosynthetic pathways. Due to this key role in controlling pathways, they are only expressed by the cell when required (non-constitutively) and are tightly controlled. This prevents the unwanted activation of a metabolic pathway, ensuring activation only occurs when needed.
The high affinity these enzymes have for their substrate helps to ensure that the substrate is able to easily bind. This is beneficial where a low concentration of substrate is present, allowing for control of the pathway through substrate concentration as well.