I disagree with those who say Finn and Rose's story was useless. It actually serves several important points beyond just reminding us of the expanse of the Star Wars universe by showing us a new world with new characters and creatures and some cool action.
1) It is an important story for showing us an example of why the First Order is so terrible and has to be taken down. We never really saw much to explain why the Empire was so detrimental to everyday people in the past. The scenes with Rose on the Casino world illustrate how the First Order impacts everyday people, turning her family and others into slaves and forcing them to a lifetime of hard labor for the entertainment and profit of the ruling class, including those who 'buy and sell weapons to both sides". This is a critical illustration and a strong reference to today's society.
In that sense, that story also illustrates how the Rebellion/Resistance is growing in that working class/slave population, through Rose, but even more through the kids working on that Casino with their secret rings displaying the symbol of the Resistance, it is kept hush hush, which may be why nobody responds to Lea's distress calls, but they are there, and they are growing. The legends of Luke Skywalker and others have helped them to have some mythical figures to believe in.
2) The same story also reinforces the point made earlier when Lea told Poe he failed by going out on his own and disobeying orders, because although he killed the main craft of the First Order, he cost the Resistance several lives.
In this story with Finn and Rose, they again go out on their own and disobey orders for heroic purpose, and for lack of trust in their leadership. It backfires very badly. Not only do they get captured and fail to destroy the tracker, they inadvertently, through Del'Toro's character, reveal the escape plan of the Resistance to the First Order, and that is the major reason why the First Order gets decimated and ends up cornered and in ruins on the salt planet.
Throughout this episode, the point is made to trust in each other if you hope to make progress. Poe and Finn try to be heroes and almost get the Resistance totally destroyed.
Luke did not trust in himself and in Ben, and Ben did not trust in Luke, which causes Ben to become Kylo Ren, and causes Luke to cut himself off from the Force and the world in general.
3) Lastly, the story of Rose and Finn culminates in Rose risking her life to stop Finn from sacrificing himself to destroy the cannon. It ends with the line from Rose that " We should protect the things we love , not destroy the things we hate. That's how we win". This is a major point in the movie and a direct reference to current events. Suicide bombers don't accomplish anything by trying to destroy the things they hate, they should ban together and protect the things they love, the purity and brilliance of their religion and culture. Similarly, in current U.S. politics, trying to destroy the other side by sacrificing yourself and your ideals does not get us anywhere, trusting in each other and protecting the things that are good is much more effective.