It stems from being “in company” instead of being at home with your family unit and the etiquette rules for a formal dinner party. When dining with members not of your household group in a more formal setting, like a dinner party, the hostess would ensure lively conversation over the meal by seating household members with new or newer acquaintances. Many factors played into this, however. Social status, wealth, relations, political scheming, matchmaking, and popularity all played into this. A hostess could make or break her reputation in society by how she seated guests at her dinner parties. For a family dinner or something informal with close friends and neighbors who all know each other, this is unnecessary. There would be no need for organized conversation, so spouses could sit together.

This is just another way Lady Catherine was showing pomposity and her over-reliance on upper-class rules and refinement. The dinner was a casual one, and Mr. and Mrs. Collins were frequent-enough guests to be considered not a “formal” guest for the Lady, nor was this a formalized dinner party. By requiring the same rules of formality for an informal dinner among neighbors, Lady Catherine was being overbearing. To the readers in the Regency period, this would have been equal parts hilarious and embarrassing. The modern equivalent would be wearing formalwear to Applebee’s.