First-naming is pretty recent, and more common in places like America and Australia than in England. It also varies regionally. My California friend couldn't adjust to the formality of Massachusetts and being Mrs. Soandso, and moved back west.

First, family names only became common with the Renaissance, but sooner for the upper classes than lower. So using them was a Good Thing, showing respect and acknowledging them.

Second, most people of the upper and middle classes were named from a list of about two dozen personal names. It's like most men of the upper classes were named George, William, Joseph &c to the point that they could shorten them in records to Geo., Wm., Jo., Th. &c. So there was no distinction to personal names , though not as bad as the short list of praenomen used by Romans or Etruscans. So using the family name of your friends was more unique to each.

Third, it was simply their way, their habit. Use of names is always a certain degree arbitrary in its rules in any culture.

Have you ever heard a couple that calls each other Mother and Father? It's a case of using the titles they expect the children to know and use. Similarly, a wife would refer to her husband as !r. Bennet because it would be too chummy to first-name him to the servants who would call him Mr. Bennet. So Elizabeth will probably call her husband Mr. Darcy to others but call him Fitzwilliam to his face in private.

In the 18th C people were even more formal. Children called their parents Dear Ma'm and Dear Sir in the better classes, if not just Sir and Madam. None of that babytalk Mama and Papa. They also differentiated between and used the formal you and informal thou quite naturally.