In a simple sense, we invoke the memory of Jerusalem following the dictum: "Let my tongue cleave to my palate if I will not remember you; if I will not bring Jerusalem to mind during my greatest joy."
Furthermore, after expressing the couple's intention to establish a home dedicated to increasing G‑d's glory, and to allow their soul connection to permeate their union, we mention the corollary of their commitment. Every Jewish home is a dazzling point of light. All these points of light eventually combine to chase away all forces of darkness, ushering in the Redemption, when Jerusalem will rejoice at the ingathering of her children.
On a mystical level, referring to Jerusalem as the "barren one" and then making mention of her children, is a metaphor for the different stages in a couple's relationship.
Originally, the two were one; seemingly an ideal state. And "they" were barren. The potential for children came about only after Eve was separated from Adam. Our ability to produce and accomplish is a result of our estrangement, and the struggles which characterize the quest of infusing two opposites with their hidden underlying unity.
Birkat Hamazon