CRA has unpleasant ways to get your attention. If they attempt to contact you and you ignore them, they are able to escalate the issue quickly.
If you have failed to explain a deduction on your return, and you ignore the letter, CRA will simply disallow the deduction. If you are asked to make payments and you do not, eventually CRA will garnish your wages or seize your bank account. Both of those results are a huge hassle.
If you talk to CRA a payment plan can be arranged. If you ignore them, you will pay more interest and run the risk of having your bank account no longer accessible to you. It is best not to ignore people who can seize your bank account.