Actually, Joey is pretty close to correct, but there are some things that need to be clarified, IMHO.
First of all, when a modern professional bowler releases the ball, he often WILL aim at the gutter, since the spin placed on the ball and the speed of the ball down the alley won’t allow the ball to start straight along the boards next to the gutter. Earl Anthony had a straight down the alley release, but often today the ball is released at the second arrow or further in, and rolls out to about the 2 or 3 board before it snaps back to the pocket.
Now, to answer the original question: Why do the balls get thrown with spin instead of straight into the pocket?
There are two reasons for a curved ball being better. One reason is that a greater angle of attack can be achieved, which is preferable as you will see. The other is that a curved ball often still has a component of spin, which helps make the pins mix up more as they are knocked off the end of the lane.
The angle of attack thing, though is the main reason for curves. To understand this best, you have to draw a mock up of the pins and the ball. A ball coming at a relatively straight angle can’t send the One or Three pins back in as effective a combination of angles, nor can it then continue through to take out the five pin and eight and nine pins as easily. A ball with a large curve will come into the one and three pins with an angle that sends the one into the two, off the side and into the four and seven, the three into the six, which one HOPES will then take the ten, and allows the ball to drive on through the five and take out the nine while the five takes out the eight, or vice versa. The most typical leaves on such a ball will be the ten, when the three sends the six around the ten, or the seven, or even the four-seven, when the two is sent too wide.