A sharp needle is used for tougher fabrics like denim or heavy canvas. A sharp needle cuts the fabric. You don’t want to cut the fabric and damage it more by using a sharp needle when it is not necessary. A ball point needle pushes the fabric asside as it moves through the fabric. When your design has 7,000 stitches and you have unnecessarily cut the fabric every time the needle pentrated the fabric, the design will not look as good as it can.
Use a ball point needle to embroider most clothing. Use a sharp needle to embroider heavier fabrics.