What is the benefit if I will intake magnesium vitamins?

It depends... First, magnesium is a mineral, not vitamin. The benefits are related to if you are deficient in magnesium- the more deficient you are the greater the benefits. Magnesium can relieve muscle spasms, menstrual cramps, headaches, constipation, asthma, anxiety, insomnia, fatigue, hypertension etc. See http://drcarolyndean.Com/magnesium_miracle/ you can also get magnesium from leafy greens, nuts and seeds.