Brian leaves the crew at the end of Furious 7 because he has chosen family life over street racing, and the scene doubles as a farewell to Paul Walker.
The action closes first, and it closes in steps. Dom defeats Deckard, then crashes his car into Mose Jakande's helicopter, and Hobbs finishes the job by shooting the grenades left aboard. Jakande is killed. Brian has already helped Ramsey shut God's Eye down, so the tracking program is neutralized, and Deckard is captured and locked away in a secret prison. Letty Ortiz then tells Dom she has her memories back, which pays off the recovery Dom was helping her with in the opening act.
Only after that does the film slow down. Brian is shown settled into a quiet family life, with another baby on the way. In the final scene Dom drives and remembers his years with Brian, and the two part ways on the road.
The production context explains why the scene is built this way. Paul Walker died on November 30, 2013, production was put on hold on December 4, 2013, and it resumed on March 31, 2014 before ending on July 10, 2014, according to the film's Wikipedia entry. Walker's brothers Caleb and Cody stood in for him, and Weta Digital added his face in more than 350 shots. Those numbers show how much of Brian's screen time had to be assembled after the pause, so a farewell that asks for a drive and a parting of the ways, rather than another stunt, fits what the crew had to work with.
Compared with the rest of the series, the structure is unusual. Fast & Furious 6 ends with the crew beating Owen Shaw, and this film begins with those wins already behind them: Brian adjusting to fatherhood, Dom helping Letty. Han's death in Tokyo, which sits right after Tokyo Drift in chronological order, sets the revenge plot moving. The loudest set pieces come in the middle, including the cars dropped by parachute into the Caucasus Mountains and the theft of the God's Eye chip from a prince's tower in Abu Dhabi, and the last scene is the quietest in the film.
My opinion: that choice is the most deliberate piece of staging in the saga, because it trades one more stunt for a quiet shot of two cars heading in different directions. Inside the story a character leaves the crew, and outside it the film says goodbye to Paul Walker. The film that follows is The Fate of the Furious. Newcomers can use the Fast and Furious movies in order guide to see where to start, so this ending lands with the full history behind it.