A Different Side of Star Wars.
The now-legends Expanded Universe took off the way it did because there was so much of the Star Wars universe that remained unseen in the feature films. One of the reasons Rogue One is an important piece of the Star Wars framework is the fact that it shows something other than the Skywalkers. So too is Solo important, for a similar reason. While Rogue One still told us of the Rebellion against the Empire, Solo takes Star Wars to an entirely different plane; the ugly underworld.
The film proves that so much more happens in the Star Wars universe outside and independent of greater galactic politics. If Star Wars is to continue beyond Star Wars Episode IX, Solo matters because we need to see more of which Solo only gives us a glimpse.