[TeamBoone]

This doesn't address why, but it estimates how many (from Ask Hal on 4/22/06):

Q: How many baseballs are used in one game? — Tom, Hamilton

A: If it's the Cincinnati Reds and either Eric Milton or Dave Williams is pitching, many more than normal. Usually, though, it is nine dozen, and if Miss Gargoyle taught me right in third-grade math, that's 108 baseballs. Mark Stowe, the visiting clubhouse attendant at Great American Ball Park, says sometimes they use as many as 10 dozen. "We lose about 18 at the end of each half inning when the players toss 'em into the stands," he said.

http://www.daytondailynews.com/sport...askhalweb.html