"I want a cheap restaurant in the north... actually make it moderate... and add Italian." Three turns, three state updates. DST keeps the slot-value dict in sync so the booking works.
In a task-oriented dialogue system, the user's goal is encoded as a set of slot-value pairs: {cuisine: italian, area: north, price: moderate}. Every user turn can add, change, or remove a slot. The system must read the whole conversation and output the current state correctly.
Get a single slot wrong and the system books the wrong restaurant, schedules the wrong flight, or charges the wrong card. DST is the hinge between what the user said and what the backend executes.