Some people need to file a tax return if they receive income that is untaxed or clearly taxed at the wrong rate. Other people file tax returns that deal with minor inaccuracies in the PAYE system. For many in this second group the amounts are small and the effort involved in the square-up is disproportionate to the amount involved.

Tim is employed by Big Co.
Each year he goes through a process to work out whether he should file a tax return to claim a small refund, or whether he has paid the right amount of tax (or maybe not quite enough) and so need not file a return.
He worries about this process because he knows that if a mistake has been made along the way, he might file a return expecting a refund but end up with a tax bill.
 

Most employers will use payroll software which will be able to draw information directly from Inland Revenue’s computer system. This will mean that errors are much less likely to arise.
PAYE is very accurate for those who work for 11 or 12 months of the year. PAYE will be treated as a final tax for this group, so there will be no need for them to file a tax return.
People who work for 10 months or less will still be able to file returns, and these rules will not apply to people like casual agricultural workers where PAYE can be very approximate.
People will still be able to file returns (and in some cases, be required to file) when they have other income or losses to which PAYE does not apply, but if they have worked for 11 or 12 months then no square-up of tax deductions for their PAYE income will be made.
 

Tim is employed by Big Co.
Tim no longer files tax returns. Big Co is using new payroll software which gets information about the right level of tax to deduct from Tim’s wages directly from Inland Revenue, so errors are unlikely to be made. Because PAYE is treated as a final tax on Tim’s wages, he knows that he’ll never have tax bills to pay.