purpose ????

They are always helpful, unless there is liquorice in the way.

If you combine a striped ball with a wrapped candy it creates a three by three blast that goes vertically and horizontally across the whole board.

Well kind of combined. You receive them together, but they are separate on the board.

Another good use of a striped candy is to use it with a colour bomb. This turns every coloured candy the same colour as the striped candy into a striped candy. This results in vertical and horizontal blasts that go right across the board.

Indeed that is a common complaint from a lot of people. When you choose to use a booster on a level it is always a gamble as to whether or not they will help you as you just don't know how many times the booster will appear.

You can create your own Striped/Wrapped candy in the game by matching four candies for a striped candy and matching candy in a T shape for a wrapped candy.

The Swedish Fish is especially notorious in this respect as for a lot of people it appears at the start of a level, and then if you are lucky it might appear at the end.