Softball Question

I know why there is a circle around the pitching rubber in Slow Pitch Softball (no stealing bases), but what is the reason for the circle in Fast Pitch?

Also, what’s the purpose of the rectangle in front of the rubber?

The rectangle is a pitcher’s lane; the pitcher has to step within the lane when making her pitch. As for the circle, softball has a so-called look-back rule governing baserunners. When the pitcher has the ball in her hand standing inside the circle, the runner must either go back to the bag or advance to the next bag; she cannot dance around to try to distract the pitcher.

do the runners not take a lead before the pitch in softball?

Nope. Kinda like Little League baseball; runners can leave the bag when pitcher releases the ball. If runners could take a lead and break early like they do in MLB, with the 60-foot distance between bases the catcher would have no chance to throw out a stealer.