Our Gemara on Amud Aleph discusses the principle behind the financial obligation of the Kesubah, שלא תהא קלא בעיניו להוציאה that divorcing a wife should not be a light matter. Since upon divorce, the husband will have to pay out a large sum, he naturally will think twice about divorce.

The Rashbah in Bava Kama (89a) elaborates on this and explains that should a woman feel that the marriage can be dismissed lightly, her general sense of security is disrupted, and their marital relations are akin to a promiscuous encounter. In other words, sexuality without a strong commitment, which includes financial security, has the emotional feel of a one-night stand.

Likutei Halachos (Even HaEzer, Laws of Matrimony, Chapter 3:11, Ones Umefutah 1:1:5, and Laws of Kesubah 1:1:4) expands on this in a mystical vein:

Every sexual union, even one that does lead to children, is still an act of procreation. These acts of joining create “souls”. (You can take that at face value, or understand “souls” as products and accomplishments of value within the relationship and the positive ramifications the bond has on family and community. See Psychology of the Daf, Yevamos 75.) The financial stability of the Kesubah, and the resultant security it provides the woman upon death or divorce, represents a physical enactment of the enduring quality of the union. Meaning to say, when there is a financial benefit, it is a theurgic representation of other kinds of spiritual flow and valences that remain in the relationship, no matter what happens in the future.

The Likkutei Halachos is adding a layer to the idea of financial and emotional security.  It is not just psychologically decent and proper, but it is enacting, sustaining, and magnifying the defacto permanence and benefit of the relationship.  All relationships are permanent.  Whether we like to believe so or not, each person we encounter affects us and we are affected by them.  That is, if we take ourselves and others seriously.