In Canada, generally speaking, there is no difference societally between a couple who is married and one who is living together but not married. Legally, however, there is a world of difference.
My family law colleagues could likely write a book on the ways that unmarried spouses have different rights from married spouses, but I am going to stress the major point in estates law: if you are not legally married, you have no right of inheritance.
There are of course support requirements if you are an unmarried spouse, but this requires essentially suing your spouse’s estate to claim a share of the assets. This can be time-consuming and costly, not to mention emotionally draining. It is far easier to have a will in place that names your spouse as a beneficiary or the executor.
If you are living with a spouse and are not legally married, you need a will.