Possible spoilers for those who haven't watched the show or Rogue One! Warning ⚠️!!!
So I was reading this article http://www.starwarsunderworld.com/2017/01/creators-discuss-adding-star-wars.html?m=1 and they stated that after talking to the developers of Rogue One the Hammerheads seen in Rebels are the same exact ships in Rogue One. It wasn't meant to be an intentional shoe horned in way to connect the two but more of a fun nod to the Rebels team and their work.
So when you're watching the show where the Princess gives them the old beat up transports one of them is critical in the battle over Scarif and the Death Star's plans wouldn't be able to be transmitted without the little ship that could.
Go little Hammerhead, Go!
Edited by Beatty