How many runs does a team score on average when there is nobody on and no outs?
The answer to that question is available by run expectancy charts, like this one provided by Baseball Prospectus. But to most people, that's just a mess of numbers, and it's difficult to follow.
Here is the same information in a chart form:
(click for a larger version)
The Red base diagrams show the runs expected with zero outs. Orange with one out. Blue with two outs. The first column is with the bases empty. Moving to the right, it shows the runs expected with 1 runner on, 2 runners on, or the bases loaded. As you can see, the more people on base, the more runs you are expected to score, and the fewer outs, the more runs you are expected to score.