What is the purpose of 30 yards circle in the game of cricket?

30 yard circle is like a limit which is used during power-plays. During the 1st 10 overs of mandatory power-play only 2 fielders are allowed to field out of 30 yards circle and for Bowling and batting power0play 3 fielders are allowed to field out of it. And during the normal course 4 fielders must be within the 30 yards circle excluding wicket keeper and bowler.