Baserunning decisions, by and large, are questions of risk versus reward. Obviously, advancing a base or two is a good thing, as it increases a team’s run expectancy. Attempting to advance also comes with some risk, as a team who is consistently aggressive in trying to advance extra bases by definition will run into some outs occasionally. (You can’t make an omelet without breaking a few eggs, right?) Yet quite often, getting thrown out on the bases decreases run expectancy more than gaining an extra base helps it. Therefore, in many instances, it’s better to just stay put and avoid running into outs.