Mantra has been used to create a level of peace in ones surroundings, to alter the effects of certain harmful substances, to cure sickness, to reach deep levels of awareness, and as a tool to reach enlightenment. It can even be used to just plain old feel good.
Many practitioners of mantra/meditation recommend that one should stick to mantras that already exist and have already been chanted by millions of people throughout time. The reason for this is that everything in this world, including the world itself, contains a particular frequency, and that frequency has a direct effect on the frequency of ones consciousness, or awareness. The frequency a mantra that has been chanted throughout all of known human existence, by over billions of people, will be far more concrete as the intention and frequency of the mantra has been saved, restored, and resubmitted to the human collective consciousness so many times that its purpose and meaning is relatively more concrete than one created on the spot. How does the mantra "Love, Harmony, Peace," make you feel? And now, how does the mantra "lay down weapons" make you feel? The first, I am guessing, is a lot more direct and understandable in regards to its affect upon your awareness.
The reasoning also has to do with the fact that ancient Sanskrit, the tongue of many well known mantras, was created with the intention of affecting human thought and consciousness on a far deeper level than strictly communication. The effects of the language on the human form have been described as magical.
I can attest personally to this great power.
Mantra has always appealed to me far greater than any other form of meditation simply because the effects upon me were always the most noticeable. I can remember a time during my sophomore year of college, walking back to my apartment from class, I decided to chant my favorite mantra Om Mani Padme Hum aloud. I remember deciding I would not stop at least until I walked into my room. I was on the complete other side of campus so I had just over a mile to walk. As I began chanting, it took effort to remember to continue, to not be distracted by my surroundings, to not be distracted by my thoughts and the eyes of onlookers.
As I continued though, it became exponentially easier to chant with every step I took, to the point that the chant completely took over, and I was no longer the one doing the chanting, my body was doing it on its own. I was watching myself walk down the road, watching the ROTC doing war games, watching people of all shapes and sizes and colors hop onto buses or cross the street, and I was watching my mouth move as I heard my voice utter over and over, calmly and endlessly, Om Mani Padme Hum, Om Mani Padme Hum, Om.... I must have looked crazy my smile was so wide, my eyes stretched open, my heart spewing with compassion for every molecule in each blade of grass and every photon in each ray of sunlight. Everything around me took on a new level of beauty and complexity. The world became exciting, thrilling, wonderful! It felt like a trip, pure euphoria, and it was all due to the repetition of a few foreign words. Awe-some
And for those of us skeptics too lazy to try something before tearing it to pieces, there is scientific evidence of the effect that simple words have on ourselves and our environment. Dr. Emoto's work is, to say the least, revolutionary! Check this shit out: Clicky! Clicky! Clicky!