It matters because it is the main thing standing between you and the mortgage amount you want. A car loan, a line of credit or even a large credit card balance all count against the total debt service ratio, which is why paying down other debt before applying can sometimes qualify you for more than raising your income would.
Your credit score interacts with the same ratios. Canada Mortgage and Housing Corporation sets a minimum credit score of 600 for an insured mortgage, and a stronger score generally gives a lender more comfort working with a ratio near the upper end of its guidelines rather than the lower end.
Joel Dyck works with local mortgage brokers who can walk through your specific ratios before you start touring homes, so your search starts from a real number. Get your hand-reviewed valuation from Joel Dyck.