Do you believe that mula bandha makes your limbs stronger? Try this and you will believe :)
Mula Bandha is a key to keeping our bodies strong and safe in the yoga asanas. As well, it helps bring the energy up the spine while keeping the heat inside the body, triggering us to sweat and release any internal toxins. Aiding in the purification aspect of Ashtanga Vinyasa Yoga.
It doesn't need to be held at the same intensity throughout the asana practice; it really depends on the intensity of the yoga pose and one's ability to do it while staying 'rooted' with the heart open, the spine elongated and the mind subtle, calm and clear!
It is also applied in pranayama practice in order to keep the energy from staying dormant at the base of the spine.