The speed arrow is another widget which can be used to help or hinder the player (like all widgets in the game). If the marble touches the square the arrow is on, it will be pushed in the direction of the arrow. Like all widgets it can only be placed on flat tiles (no slopes).
This is probably the simplest of widgets, but one which has many uses. It can be used to push the marble off a narrow walkway, or to help it gather enough speed before rocketing off a slope.
Hopefully, this serves to drive home the point that, for the most part, widgets are neither good nor bad. They just are. It is up to the creator of the level to decide how they are used. Hopefully this freedom will yield interesting results from both official levels and user-made levels.
Of course there are widgets such as triggered lasers which are pure obstacles to be dealt with. It will be interesting to see if someone finds a way to make these lasers helpful to the player. 🙂