As with any sport, having equipment made for you specifically, will enable you to perform better, learn quicker and be more comfortable as the equipment is tailor-made. In bowling, this translates into a ball that fits your hand. This allows you to be more accurate, improve your game quicker and with less fatigue, because you are using your strength to guide and propel the ball instead of using some of that strength to just hold own because the ball doesn’t fit your hand. This will also allow you to bowl more games, because you don’t have to grip the ball to hold on.
If you are a more accomplished bowler or want to become one, you will need your own equipment to know that when you throw the ball correctly, it will perform the same with each delivery.