Down Dog

Considered a basic pose, this asana is not only a "hub" for many other asanas it is beneficial in many ways to the body.  Heels to the floor is NOT important.  Lengthening the legs is.  If your heels do not reach the floor, it's fine.  Just please don't stand on tippy toes nor force your heels to the ground.  Neither are helpful in this pose.

1.  It's a mild inversion.  Your head is below your heart, so you reap benefits of turning the  body upside down a bit.
2.  Strong shoulders and upper body.  Putting this much of a person's body weight onto their arms is a great way to build strength in the entire upper body and torso regions.  Keep the shoulders stable and engaged to prevent injury.  If the wrists becoem sore, consider doing dolphin pose which is very similar to Down Dog but on the forearms.
3.  Imporved posture.  Working on your Down Dog will imporve your posture.  Keep the butt pushing up towards the sky and to the rear will lengthen the spine.  You do not need to have legs completely straight.  Protect your hamstrings.  Try to keep the neck long and ears between the biceps.  Posture, Posture, Posture.
4.  Strengthen bones.  Weight bearing activity strengthens bones.  This asana falls into that wonderful category.

---