Robert Downey Jr. is playing Doctor Doom in the upcoming Marvel Cinematic Universe (MCU) films "Avengers: Doomsday" and its follow-up "Avengers: Secret Wars," marking a surprise return to the MCU after more than six years since he last played Iron Man. The casting decision was made by Marvel Studios chief Kevin Feige, and the idea to cast Downey Jr. as Victor von Doom existed even before he was cast as Iron Man. Downey Jr.'s Doctor Doom is expected to be a distinct character with no direct connection to his Tony Stark/Iron Man role, meaning Downey Jr. will portray Doom behind the iconic mask without showing his face, separating the two characters. Doctor Doom is a brilliant yet tyrannical villain, an evil scientist and sorcerer who is the nemesis of the Fantastic Four and one of Marvel's greatest antagonists. The MCU is reintroducing this character played by Downey Jr. to leverage his star power for a pivotal villain role in its future storylines. There has been speculation about possible connections between Doom and Stark due to their shared actor, including theories of alternate realities or variants, but Downey Jr. has dismissed the idea that he is playing an evil Tony Stark variant. Instead, it will be a separate, different character role, with a complex backstory that Downey Jr. has been involved in developing.

In summary, the reason Robert Downey Jr. is playing Doctor Doom is to make a significant and surprising casting choice that leverages his iconic status within the MCU, introducing a major Marvel villain with a fresh take while distinctly separating this role from his Iron Man legacy.