Doing it online helps to avoid having your correctly entered paper routine messed up by CRA casual / temp workers.

If you are paid bi-weekly you may have to pay more EI or CPP tax some years. One year I got shafted that way and filed my tax correctly, including the exceptional 27 pay period premiums and corresponding deduction amounts.

The poorly trained droid who reviewed it incorrectly reduced both amounts. When I queried Revenue Canada about they agreed that their droid made a mistake, but said that the difference was too small to send me the Refund Difference, and did not offer to carry the balance over to a future tax year.

I also sent my paper refund in the same envelope as my wife's tax return one year. Each with a separate paper clip. They cashed my tax payment cheque, but claimed that I had not filed a return until I contacted them and told them to review the scanned image of my wife's return to see if their droid had failed to enter them as 2 separate returns.