We do it to make the defense have to work to get an out. Even crappy teams can check the runner at 3rd and throw the ball to first and get an out. We do it with R2 and R3. Both runners are moving on a ground ball. If R3 is a dead out, then his job is to get in a rundown until the hitter can get to second. R2 moves up to 3rd, and the worse that happens is you were where you started, but much harder out for the defense to get. Many times the coaches are yelling to run R3 back to the bag since it is now occupied.... When they do and R3 is safe back at 3rd, R2 takes off headed back to second. If R2 makes it back to second the hitter heads back to first.
Many times R3 scores with no throw to the plate, and then sometimes it looks like a cluster, but we wind up scoring more times than not. Then again if we don't score we still have runners at 2nd and 3rd, and the defense had to work to get that out.