Why use granular synthesis ?

I finally found out in the “Digitale Klangsynthese und Klangverarbeitung II” (digital sound synthesis and processing) lecture¹ I take :

Granular synthesis gives synthesized sound texture, roughness and dimensionality.

I like to find out the why, before I get to the how.

¹The lecture is at the icst.net (Institute for Computer Music and Sound Technology) in Zürich, Switzerland.