It's a left-over bit from a dropped plot point. The following quote from Rowling I found at the Harry Potter wiki, originally from Pottermore:
Florean is a descendant of Dexter, and I originally planned Florean to be the conduit for clues that I needed to give Harry during his quest for the Hallows, which is why I established an acquaintance fairly early on. At this stage, I imagined the historically-minded Florean might have a smattering of information on matters as diverse as the Elder Wand and the diadem of Ravenclaw, the information having been passed down in the Fortescue family from their august ancestor. As I worked my way nearer to the point where such information would become necessary, I caused Florean to be kidnapped, intending him to be found or rescued by Harry and his friends.
The problem was that when I came to write the key parts of Deathly Hallows I decided that Phineas Nigellus Black was a much more satisfactory means of conveying clues. Florean's information on the diadem also felt redundant, as I could give the reader everything he or she needed by interviewing the Grey Lady. All in all, I seemed to have had him kidnapped and killed for no reason. He is not the first wizard whom Voldemort murdered because he knew too much (or too little), but he is the only one I feel guilty about because it was all my fault