In many cases, you are required to file your taxes by law even if you won't owe. If it's anything like in the US, it's quite possible your employer is not taking the right amount and you may owe more or may even be in line for a return.

http://www.usatax.ca/Pages/filing_requirement_taxes_canada.html

If you took advantage of options like a home buyers plan (HBP) you definitely need to file since you must designate how much of the plan to repay. Your employer does not know about what you do with your money so cannot take this into account for the withheld taxes. If you do not report repayment of the HBP it will be treated as a withdrawal from your RRSP i.e. additional income for that tax year.

As a Canadian resident, the simple answer to your question is "yes" Having worked as a tax auditor and as a Certified Financial Planner, you are required to file an income tax return because you have taxable employment income. All the employer is doing is deducting it at source and remitting it on your behalf. That does not alleviate your need to file. In fact, if you don't file you will be subject to a no filing penalty. The one aspect you are missing is that taxpayers may be entitled to tax credits that may result in a refund to you depending on your personal situation (e.g spousal or minor dependents). I hope this helps.

While you are required to do so as others have said, it's actually in your interest to do so. In a recent article at GlobeInvestor, Tim Cestnick discusses the benefits of filing tax returns for teens. This situation may or may not apply to you but the message is the same. The main benefits are (1) create RRSP contribution room and (2) be eligible for GST/HST credits and other possible one-shot credits (think oil royalty surplus cheques in Alberta). Excerpt:

You see, when Lincoln was 14, he filed a tax return and reported $2,000 of income that year. He paid no tax thanks to the basic personal tax credit, but he created $360 of RRSP contribution room that year. Beginning in 2003, Lincoln started working part-time in his father's business. His father agreed to pay him $6,000 each summer to work in the business, to help save money for university. Lincoln didn't pay any tax on the money he earned in those summers because his basic personal tax credit was always higher than his earnings. In addition, Lincoln added to his RRSP contribution room simply by filing a tax return each year.

Yes, you have to file a tax return in Canada. Non residents that have earned employment income in Canada are required to file a Canadian personal income tax return.

Usually, your employer will have deducted sufficient taxes from your pay-cheques, resulting in a tax refund upon filing your Canadian tax return.

You will also receive a tax credit on your US tax return for taxes paid in Canada.