Even if you and your partner never planned on getting married, that doesn't mean you would never live a life nearly identical to that of a married couple. Maybe you co-own the home you both lived in. Or perhaps you have children together. Like married couples, if you haven't prepared for the potential dissolution of the relationship, you may find yourself having a difficult time logistically following a breakup. Also, if your partner dies, it would benefit you to have an estate plan in place since you won't be the default beneficiary.
This is where it would really serve every non-married couple who is making serious strides with each other to put together a plan or agreement. This could lay out how down payments were or will be made on your home purchase, who will make mortgage payments or what contribution each person will make toward the payments. Also, document how taxes are paid and who gets any returns, since you won't be filing as a married couple.
If you have children, you will want to have a default co-parenting plan in place that you update regularly. Having all of these records and plans in place at the start of a non-marriage can help you both avoid unnecessary hardship should your relationship terminate.
A family law attorney who works with divorcing couples can prove very insightful when unmarried couples need to figure out how best to make a clean split when assets and property have become intermingled. Division of assets are difficult during divorce and can be even harder when a couple have not married and have not planned for the possibility of a split.