The missions that improve your score multiplier serves as half of the accomplishable feats in Subway Surfers. On the missions window, be sure to check the achievements tab as it holds a variety of achievement objectives that very much serve as milestones in relation to your overall progress in the game. Unlike the mission objectives that are replaced with new ones after completing a set of 3, achievements provide a continuous goal for you to target, with each milestone rewarding you with extra keys.
Although keys can be obtained in a number of ways, every free key you can obtain should be considered as an opportunity that you should not miss. Note that if you are ready to expend your keys for that record-breaking run, each usage will consume more and more keys at a time. In this sense, going for a perfect run requires massive farming for keys across all possible sources and each one should be fully maximized.