There were 2 supplements for the previous story line:
- Adventure Supplement (~55pages) > PHB/MM/DMG stuff that is not in Basic
- Player's Companion (~25pages) > entirely new character stuff (races & spells) that's neither in Basic nor in the PHB
Clearly the purpose of the first is to let people who have only Basic (or are missing some books from the core 3) to have everything they need to buy and play the adventures. This was a very good move from WotC and consistent with the idea that people could buy any one
book (including adventures) and be able to play the game without more purchases. A similar document was released for the ToD storyline the year before. It makes sense to do the same for the RoD adventures, even if it means that it will overlap in content with the previous.
The purpose of the second is to introduce new character material. Honestly I was surprised they did that, because I thought this kind of material is kind of "premium" and comes with the printed book only. But obviously it was nice by WotC to make it available to all
There was no equivalent of this for the ToD storyline, most probably because the whole 5e had been just released so all character material was new, and there was no need to add more already at this stage.