Most of us have at least one ancestor who was married more than once. Normally we do not descend from each of their spouses and we tend to focus on the spouse from which we descend.
Doing this may cause us to overlook information. Researching all our ancestor's spouses may provide more information about the ancestor.
Archibald Kile was married three times. The first was in the 1830s in Ohio to the woman with whom he had all his children. He married twice in Illinois, both times when he was in his 70s. Searching the records of these marriages located marriage applications which provided the names of Archibald's parents. If I had not located the second and third marriages of this ancestor, I would have missed a great place to learn about his parents.