In the Air
When the ball is in the air, whether hit or thrown, it will be spinning. (We will ignore the knuckle ball since young players cannot throw it properly since their hands are too small and they cannot control it without years of practice.) A spinning ball will curve.
The faster the surface of the ball is traveling with respect to the air over it the harder the molecules of air will strike the irregularities on the surface of the ball. This increases the pressure on that side of the ball. Since the ball is moving through the air in some direction, if the axis of rotation of the ball is perpendicular to the line of flight of the ball, one side of the ball will have not only the velocity of the center of gravity of the ball but the velocity of the spin as well. The other side of the ball will have the velocity of the center of the ball minus the spin speed. This can make a considerable difference in the relative velocity of the two sides of the ball.
The ball will be pushed by air pressure in the direction of the “fast” side of the ball. So long as the ball is not perfectly smooth, it will not matter whether the ball’s seams are going through the air sideways or lengthways. Only the relative velocity of the two sides of the ball matters. Thus, if the baseball is spinning like a football with a tight spiral the ball will be curved in flight only by gravity and wind. It is impossible for a 12 year old baseball player to make a throw that does not curve. They would have to put enormous backspin on a very fast pitch and have the axis of rotation almost exactly perpendicular to the line of flight to counteract gravity.
Almost all fly balls to the outfield have backspin like a fastball. Therefore a line drive will tend to “float” over the fielder’s head. Also, pitches that are “pulled” down the line or “slapped” to the opposite field will tend to curve toward foul territory. The left and right fielder should play fly balls near the foul line to curve toward the foul line.
Very high popups spend so much time in the air that their spin slows quite a bit. This lets them become knuckle balls if the spin gets slow enough. That is why high popups can be very hard to catch. 12 year old players cannot hit the ball high enough for this to be a problem. The popups they see still have considerable spin and tend to float further toward the outfield than the player expects. That is why so many infield popups fall just behind the infielders.
On the Ground
When a fly ball hits the ground its spin is translated into movement. Therefore all popups and fly balls try to bounce toward the nearest foul line. Grounders have “top spin” in which the bottom of the ball is rotating back toward the bat. This allows the ball to hit the ground without losing as much horizontal speed as it would if not spinning. The first time the ball bounces its spin will match its horizontal velocity and the spin will no longer affect its bounces. The top spin of a grounder will not exceed its horizontal velocity unless it is hit into the ground close to the batter. A normal grounder will not speed up from spin no matter what the playing surface is.