Hand checks slow down a guard that tries to drive and allows a defender to play tighter defense.The problem is that Americans are used to the way fouls are called in other sports.
Basketball- Ref will call a foul for a handcheck on a defender that has very little impact on the offensive player. Or will call a foul away from the ball that has zero impact on a team's ability to score a basket in that moment. Or a call for stepping over the halfline (over and back) that doesn't really impact the play. But those are the rules and you'd never hear anyone say "well, he just stepped over the midcourt line by a few inches, so let the game be played."
A foul away from the ball can keep player from cutting to the basket, getting open, or defending properly.
Over and back needs to be called as it allows a team to trap a player at the half court line, usually near the sideline.
Hey, my daughter is playing varsity basketball. Need to stay up on these rules as well.