Andor begins five years before Rogue One, with a somewhat lost Cassian Andor (Diego Luna) who hasn't yet ed the Rebel Alliance. Through flashbacks, Andor tells how Cassian, a.k.a. Kassa, was rescued from his home planet of Kenari by his adoptive mother, Maarva, before the Empire wiped out the Kenari people. As an adult, Cassian is still hoping to find his biological sister.
While Rogue One is, ironically, the most straightforward war film of Star Wars, Andor is the more isolated story of a refugee. There have been fights, but no real battles. Rogue One featured Darth Vader and frequent mentions of the Force, but there are no references to Jedi, Sith, or the Force in Andor. It would have been easy to make Andor just like the financially successful and critically-acclaimed Rogue One, focusing on Cassian doing missions for the Rebel Alliance, but Disney made a smart move by going in a different direction.
Why It's Important Andor Is Different From Rogue One (While Setting It Up)
Andor is still grounded in rebellion, just on a smaller scale than Rogue One. Cassian is acting out against the Empire—he stole an Imperial Starpath Unit—but he has no official training or backup. Starting Andor this early in Cassian's rebellion story bolsters his character arc in Rogue One. It allows him to grow more across the five years Andor seasons 1 and 2 will cover, leading naturally into Rogue One without contradicting it.
In Andor, Cassian is in the relatable position of someone who wants to do more to help downtrodden people like him, but not knowing how. This is opposed to his position in Rogue One, when he's an experienced rebel—and more cynical. Andor will show his development into that character, as well as the Rebel Alliance's development in the five years before destroying the Death Star. In a way, Andor was able to mold Cassian from nothing, focusing not on who he is in Rogue One, but who he was before that. Star Wars hasn't been able to do that with other live-action shows focusing on well-established characters like Boba Fett and Obi-Wan Kenobi.
Plus, the story of a refugee—even one in a galaxy far, far away—is always relevant to the real world. Andor tells a story about a relatively small character in the grand plot of Star Wars, but it's an important story. Had Andor attempted to replicate Rogue One's success, it likely would have fallen flat. Instead, Andor acts as its own story, while still complementing its pre-existing sequel.