The hash marks are used for ball placement prior to most offensive plays so more of the game can be played in the middle of the field, which makes the game more wide open. If the ball was placed 20 feet from where it went out of bounds rather than on the closest hash mark, offenses would be restricted to one open side of the field for many of their run and pass plays. In other words, they would have to run or pass to the right or the left, and wouldn’t have the option to do either. But, when teams run the football and the ball carrier is tackled between the hash marks, the ball is declared dead at that spot and generally is placed where the ball carrier was tackled and stopped.
An important thing to remember is that an incomplete pass is returned to the spot of the preceding play, not where it actually goes out of bounds or where the quarterback was standing when he threw it.