Bowling to a leg-side field is usually seen as a defensive move.
It can be used to stop the flow of runs, particularly against a batsman who's especially strong through the off-side.
Sometimes, the fielding team will use a deep square leg to try and tempt the batsman, with short-pitched bowling that could be pulled or hooked in the air.
However, because of a law introduced after the "Bodyline" Test series between England and Australia in 1932/33, only two fielders are allowed behind square on the leg side. If more than two fielders are in that area a no-ball will be called.