Research on Magnesium shows that it absorbs readily through the skin and many do not absorb through the gut because of low stomach acid (which may be due to anti-rreflux type meds). It perfoms over 400 functions in the body including:
 
Nerve Function
Bone Growth
Blodd Pressure
Fighting Depression
Muscle Function (twitches etc)
Energy Release and Control (including relaxation/sleep)
Indigestion