Star Wars: The Last Jedi - 39 Things We Learned From The Director's Commentary

7. The Cannon

The climax of the movie comes on Crait, with the First Order rolling out a battering ram cannon to break down the barrier the Resistance is hiding behind. This is, says Johnson, where Poe learns the lesson that you can't blow all the best pilots on a suicide mission and that you have to see the bigger picture.

The idea is that lesson crosses with Finn, who finally sees that the Resistance is worth fighting for, which is then crossed with Rose who says we win by protecting the things we love. Johnson needed the audience to buy that Finn was going to commit suicide for at least a moment, so Rose saving him would be such a relief and show how far Finn has come in this movie.