"ballpark (figure|estimate)" Meghatározása és Jelentése

What is the origin of the idiom "ballpark figure" and when to use it?

The idiom "ballpark figure" is derived from the idea of a ballpark, where sports games are played and spectators often make rough estimates of the number of people in attendance. It is often used to provide a rough, approximate numerical value when an exact or precise figure is not available or necessary.