When creating a new form using the New Form Wizard, the developer may want the available fields listed in the order of creation rather than in alphabetical order. There may have been a logic to the order they were created and a significant number of fields to sort through to place on the form.
The New Form Wizard defaults to the Master Table option when it is first invoked and this lists the fields in alphabetical order.
When the Related Tables option is chosen, the fields show in the order they were created.
If the developer requires fields from an unrelated table on a form, they can temporarily create a relation so the fields from that table will be available for selection in their creation on order.