Even if you haven't paid any tax during the year (i.e. through withholdings or directly), it is beneficial to file a tax return because
A) you can carry forward all non-refundable tax credits to future years.
B) you can get a tax refund for your RRSP contributions.
C) you may be eligible for a tax-free refundable GST/HST credit.
D) it is required by law, if you are over 18.