if you like drawing living things (and you care about proportion, realistic renditions) studying the underlying musculoskeletal structure helps.

i just try to capture something fleeting. i identify the most salient element and try to communicate that in my drawing. the most useful exercise in that book imo was the technique of trying to draw something once, then turning it upside down and trying again. ("disorienting" the object trains your mind to better identify spatial relationships.)