Kit Tyler's relationship to William Ashby is a complex one that initially operates around the social and socioeconomic customs of the time. Before we dive into this relationship, it's important to remember that Kit's experience of Puritan New England requires significant adjustment on her part after leaving behind her luxurious life in Barbados. Kit--who was used to the privileges that came with financial security in a non-religious environment--must entirely change her lifestyle, from learning how to dress pragmatically and modestly to becoming used to doing her own chores. Thus, when William--the wealthiest unmarried man in Wethersfield--shows interest in her, she chooses to be open to the courtship. Although she does not find William to be an interesting suitor or one with which she could develop any true romance, she does consider marrying him for the social and financial perks that he could provide: an elevation from her average station in life, a reprieve from physical labor, and a chance to regain some aspects of the lifestyle she had lived in Barbados.
Ultimately, Kit does not end up with William (a smart move after he abandons her when she is put on trial for being a witch!), but her evaluation of him as a potential husband is still a reasonable and significant component of her character's growth over the course of the novel.