As I understand it, they are a person that links you to an established pedigree..
in Matts case Sophia Anson (not, in my opinion, her father as stated in the program).. Its that Anson name that immediately caused Lord Lichfield and Shugborough Hall to spring my mind when I heard it, before Sir George Anson was even mentioned
Daughters of wealthy or famous men often go unnoticed as a promising ancestor, because of their name change, they provide a gateway to a whole new genetic line
eg "John Smith and his wife Mary" is not very inspiring
but "John Smith and his wife Mary, the daughter of Sir Giles de Berkeley" is a whole new ball-game