I heard a wonderful explanation from David Swenson regarding the importance of pranayama, breath bandhas in Yoga and their relationship to Asanas; He used the analogy of an sculptor. To build a statue an artist needs and uses both heavy duty tools and detail-oriented tools. Smash away first with a hammer and large chisel to get some ways to where you need to get…and then use the smaller picks, chisels and scrapers to bring out the detail. You need both
You could use the subtle tools alone but that would take a very long time. You could use the heavy duty tools alone but the end product won’t have the subtle detail you need. The Asanas are the heavy duty tools. Pranayama and Bandhas the subtle tools. The former prepares the body and the nervous system for the power – the prana – that will be released through pranayama, and then contained and channeled by engaging the bandhas.