pukunui
Legend
This. I don't see these sorts of stories as "filling in the gaps" so much as just "playing in the sandbox". Yes, they're fleshing out the stories of known characters like Saw Gerrera and Mon Mothma, but it looks like they're also introducing new ones, like Stellan Skarsgård's Luthien.The way I see it, the setting of the Rebellion era before the Death Star is just a good place to tell stories. Not because there is something particular about that era that needs to be "clarified" or anything - just because fighting an oppressive regime is a neat thing to show.
Also, Rogue One is one of my favorite Star Wars movies. I think I might even enjoy it more than A New Hope. It's not perfect, but it hits all the right notes -- it both feels like class Star Wars and like the classic WW2 adventure films Lucas drew on for inspiration (like Bridge over the River Kwai and The Guns of Navarone and such).