Why chant “Nam-myoho-renge-kyo” instead of any other words? Do the words themselves have power?

All words have power. Words can make people happy or sad, angry or loving, and so on. Language has a more profound effect on our lives than we realise. Nichiren Daishonin says: “They say that… if you so much as hear the words ‘pickled plum,’ your mouth will begin to water. Even in everyday life there are such wonders, so how much greater are the wonders of the Lotus Sutra!” (“The Daimoku of the Lotus Sutra”, WND-I, pg 141).

Historically, Myoho-renge-kyo is the title and essence of the Lotus Sutra, the highest of Shakyamuni’s teachings, as translated from Sanskrit into ancient Chinese characters. Nichiren Daishonin placed “nam,” a Sanskrit word meaning “devotion”, in front of these characters; so that in simple terms Nam-myoho-renge-kyo means devotion to the ultimate Law of the universe. Each of the characters themselves contain extremely profound principles of life, however, and together they express how everything in the cosmos works in one harmonious relationship.
Of course this is extremely difficult to believe and understand, but that does not mean it cannot be understood, or that chanting this phrase does not have a profound effect on our lives.
Scientists and mathematicians use formulas and language which are quite incomprehensible to most other people, but they can convey even the most difficult concepts to each other with accurate use of these expressions. Daisaku Ikeda, president of Soka Gakkai International (SGI), says: “In the same manner, when we chant Nam-myoho-renge-kyo, even if we don’t understand its profound meaning, we can tap the condition of Buddhahood. Our voice chanting Nam-myoho-renge-kyo permeates the cosmos and reaches the life condition of Buddhahood and all the Buddhas in the universe. It also penetrates our lives, enabling us to unlock the palace of Buddhahood, or the supreme life condition of eternity, happiness, true self and purity. It is the same as music that, without any explanation, reaches and filters into people’s hearts, calling forth a sympathetic response from them.”
The most important point to remember is that everyone who chants Nam-myoho-renge-kyo can draw out of themselves the state of Buddhahood which influences all the other states in our life and guides our actions in the very midst of our day-to-day struggles and problems. Although it is very difficult to believe at first, Nam-myoho-renge-kyo is the essence of everyone’s life, and when we chant this phrase it affects us in countless positive ways, from the very core of our lives.
The extent to which we can benefit from chanting depends not on the power inherent in the characters Nam-myoho-renge-kyo, which is limitless, but entirely on how much sincere effort we make when we practice. The natural Law of life is in everything, but only by chanting Nam-myoho-renge-kyo and taking action in our daily lives can we tap or activate it, and so enable it to work for the happiness of ourselves and other people. Buddhism talks about the principle of the “four powers”: the power of the Buddha, the power of the Law, the power of faith and the power of practice. The first two are embodied in the Gohonzon, but are only activated by the powers of faith and practice of the person who chants to it. In other words it is the power of our faith and practice which determines the extent to which the power of the Gohonzon can appear in our lives. 
Josei Toda, the second president of the Soka Gakkai, said that it is vital to understand this point:
“I often hear new members complain that they have not yet received any benefit. I find the majority of those people are preoccupied solely with getting benefit and pay no attention to strengthening their own powers of faith and practice. The statement ‘Knock and it shall be opened unto you’ serves as a good illustration of the four powers. For example, if one exerts his powers of faith and practice to a factor of 100, the powers of the Buddha and the Law will also be manifested to the power of 100. Likewise, the powers of faith and practice exerted to a factor of 10,000 will manifest the powers of the Buddha and the Law to a factor of 10,000. We should fix this principle in our minds and dedicate ourselves to faith and practice without reservation.”