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Why do I need to apply for TRV when I have my POST GRAD WORK PERMIT?
Work permit and TRV are two different things. I assumed you asked about TRV because that's what is put on your passport. If you want to extend your TRV, you must apply and pay the fees $100 - After your TRV application is approved (It will be since you've a valid PGWP) you have 30 days to send your passport to Ottawa so the TEV is stamped.

IRCC Website:
A work permit or study permit is not a visa.

We won’t automatically issue you a visitor visa if you extend your work or study permit. If you need a new visitor visa, you may need to submit a separate application.

You can apply for a visitor visa from inside Canada if you meet all the conditions below:

  • you’re already in Canada
  • you hold a valid study or work permit
  • you want to leave and return to Canada in the near future
  • your existing visitor visa is expired or was valid for only one entry

Good luck!
 
Wait so through online application if it gets stamped then still you apply for TRV? I do not understand.
PGWP is a physical document

TRV is a stamp on the passport. You have to apply online and wait. Once TRV is approved, you can send your passport. Best of luck