If you are married or living common-law and are not preparing a dual (or "coupled") return with your spouse or common-law partner, you must fill in a Net Income for your spouse even if you are not claiming a married amount. The program adds your spouse's net income to yours to determine your net family income. This amount affects many credits and deductions, including the GST/HST credit, Canada Child Tax Benefit, spousal amount, and certain provincial credits.
The CRA requires you to provide this information.