They do throw the balls out that have hit the dirt after it has been hit. Fielders finish the play and throw it back to the pitcher, and he will throw it back to the catcher to give to the umpire.
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Even though baseballs are usually rubbed with mud in order to break them in before a game, the mud doesn't exactly stay on the Baseball when a pitcher uses it. Sometimes if a baseball bounces in the dirt and an umpire sees it clearly then they'll usually request it for it to be thrown out of play so that a new baseball can be used, this is done because foreign substances (such as dirt) on a baseball can trigger how it moves depending on what pitch the pitcher throws.
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The above answer is pretty much correct. MLB wants the balls to remain scuff and mark free. That is why the average baseball in a major league game lasts only 9 pitches before being taken out of play. And the mud that is used to rub up the balls before the game isn't mud in the sense you would typically think. It's not wet like naturally occurring mud. It's purpose is just to take the sheen off the ball...not to dirty it up or marr it in any way, and it does nothing to "break in" the ball. If a baseball were to become "broken in," it woul dbe removed from play.
because the dirt creates scuffs, scrapes, dirt, and other marks which change the air flow over the ball where the marks are helping the pitcher by creating more curve, drop, or other actions on the ball making if more difficult for the hitter to hit. It also keeps the ball white and visible allowing the batter to see it and avoid it if necessary.
Yes because if it is dirty, they pitcher will not be able to get a grip. BUT, IT IS NOT LIMITED TO THIS, the umpire usually determines a ball change because the ball being scuffed on a errant pitch can actually produce scuffs that the pitcher desires-alot of pitches are thrown with better spin, curve, or travel when the finger has a grove to set itself in. Several major league pitchers over the years have even been caught with sandpaper in their gloves to scuff a ball over and over. Also, balls are "pre-dirtied" before a game with actual specially designed product that a ball boy or clubhouse person uses to rub on the baseballs (about 100 or more for ONE game) to reduce glare for the batter and catcher and also so it's NOT so slippery for the pitcher.
In today's games, most baseballs are removed from play in both cases. You just may not notice the ball being removed after and grounder, but it normally is.