Pay And Display car parks rely on employees who check that vehicles are covered by a payment by looking at the ticket on display. With telephone payment they do so by checking the registration number against the record of payments. The employees need a data connection to do so but their phones are easier and cheaper to update than a machine - and less likely to be vandalised.
The same switch was made here years for the council's on-street parking and residents' parking permits. Enforcement officers now using ANPR on their phones.