It's kinda the same thing as the entire prequel trilogy. We knew, in a general sense, the backstory of Anakin Skywalker, how he was betrayed and murdered by Darth Vader (from a certain point of view), but seeing how it all happened was an opportunity for some good storytelling. Okay, that's not what we got, but the idea of filling in the blanks is not a bad idea in and of itself.
The Kessel Run could be really, really cool to see, an amazing FX spectacle, and we'll finally get the canonical explanation as to why the run itself is measured in terms of distance, not time. The game in which the Falcon changed hands could also be very cool. We imagine them playing poker or something, but we have no idea what the game was, do we? And hopefully also a random adventure or two involving Han. I personally would like to see the gig where he ended up dumping his load of Spice, which got Jabba so upset, because I'm a huge Dune fan (the books, not the movies) and always liked the Dune shout-out.
Again, it could be very cool to finally get all these details. Is it necessary? No. But for those who can't get enough Star Wars, it'll be out there, and presumably the "quality control" folks at Disney will be harder on this story than Lucas was on the prequels, and we hopefully get something good.