It's not a need. It's a marketing thing. When a travel agent is searching for flights for a customer - let's say, in your case, from DFW to LHR. Surely, any nonstops on AA & BA would appear at the top of the list. Then, under that, would appear any flights - like your DL29 would appear next, that make a single stop with the same flight number. Then connecting flights will appear under that.

So, it pushes DL29 to near the top of the list for VISIBILITY, but only people "in the know" actually know that it's likely a different plane altogether - - AND - - that they likely won't hold the connecting aircraft if the first leg is late! If you search, you'll find many posts about this "syndrome"!