Angles have a special meaning with sprites. If all angles are at zero, the sprite will stand upright and horizontally face the camera. If its pan or tilt angle is nonzero, the sprite is oriented in world space according to its angles. If pan and tilt both are 0, but the roll angle is nonzero, the sprite is always perpendicular to the camera. This is useful for spherical objects, like fireballs or explosions. For an angle to be nonzero it's sufficient to set it at a small amount, like 0.01.