Mantras are integral part of the Yogas. They come under Mantra Yoga and Japa Yoga. There are several reasons why we enchant a mantra. But all leads to thee [melt and merge with almighty].
Every mantra starts with pranavam [Ohkar]. Having Bija Mantra as a integral part of the Mantra, makes the mantra powerful. Any mantra without bija mantra is like a vehicle without fuel. To understand mantra you should know the knowledge of Pranavam & Bija / Seed Syllable.
Meaning of Mantra:
Mantra is a Sanskrit word made up of two verbs, Man & Tra which in Sanskrit "Mananat Trayate", just by chanting one can save himself in the universe that is achieve the ultimate in this material world.
Man : The person who enchants it [The person who does Mananam]
Tra : That which protects