This must be why the Dreamcast has an IDE pinout on the motherboard that can be tapped into with Dreamshell to load games. They were thinking about giving a HDD early on, along with the implemented built in modem and DVD, but Bernie Stolar said he was forced to pick one of those three things and a modem won out in the end. So it was a cut feature releatively late enough into the Dreamcast's development process that the pinout is still on the retail console. Don't forget there was actually a prototype cable box with built in Dreamcast that also had a HDD hooked up to it for downloading games to. Anyways, Tom Charnock of Dreamcast Junkyard interviewed Bernie Stolar about this like 2 years ago, and he had this to say about the subject.
Tom Charnock; After you left SEGA of America, did you continue to keep an eye on how the Dreamcast was doing? It was effectively your baby, after all!
Bernie Stolar; Yes. More money could have been spent on product. I said this from the beginning. There were three things that I wanted in Dreamcast: an online network (for multi-player and digital downloads), DVD support, and internal storage. I had to argue for everything. At one point, I had to ensure the modem didn’t get dropped from the US version. Online was most important to me, so I chose that over DVD and internal storage because my plan was to add those later. With regards to internal storage, I also began discussions with an early DVR company about releasing a cable box with the ability to download our games, especially Dreamcast titles, as the plan was to deliver Genesis, Master System and even Saturn games as well. Similar to the way LodgeNet worked in hotels.
The infamous PACE Dreamcast set top box
Tom Charnock; In terms of the Dreamcast hardware itself, it was a very innovative system. Are there any features you know of that were planned but abandoned before they could be implemented? We’re aware of the VMU MP3 player and the SEGA/Swatch partnership et al, but were there others?
Bernie Stolar; More online play, DVD support, and internal storage (including the DVR games on-demand discussions). I also pushed hard for a dual joystick controller, similar to what we did at PlayStation. There’s a reason the DualShock is still used to this day. I chose online functionality over DVD playback and internal storage, because I was forced to pick one of the three due to budget limitations. I would have preferred obviously to include all three from the get-go or at the very least, online play with a DVD drive. We were also in talks about Grand Theft Auto III, Warcraft: Online (as it was known at the time), Max Payne, and other titles coming to Dreamcast, and this would have helped tremendously.
Source:
http://www.thedreamcastjunkyard.co.uk/2018/03/an-interview-with-bernie-stolar.html
So this seems to me why possibly it has it on a prototype system?