I'd say it's basically for when you want to take stuff away in big chunks or small chunks. I'd also say it for starting stages of the sculpt when you are still finding the shapes and it's all malleable and you can take liberties and hack and slash stuff, for maybe defining silhouettes. I'd also say that it's not really a science, there's no right or wrong per se, you'll begin to discover for yourself what tool in the tool shelf you feel most comfortable with, so just experiment and see how it goes. I remember Kent trimming big time at the initial stages of the Sci-Fi Helmet course:
https://cgcookie.com/courses/modeling-a-sci-fi-helmet-in-blender