If your spouse is of child-bearing age and a woman, then it is possible (even if unlikely) for them to become impregnated by someone else. This is more likely if you are separated. In Massachusetts if a woman is married within 300 days prior to the birth of a child, then her spouse is legally the father of that child, whether or not he is the biological father. Being a legal parent comes with both obligations (child support, health insurance) and benefits (visitation, relationship). But if it's not your child do you want that responsibility?
It is possible to undo this legal presumption but it usually requires knowing who the biological father is and suing them. Now because you waited to get divorced you have two lawsuits instead of one and another party involved in the dispute (the biological father).
Read Reason #4: Remarriage.