Many times in Western society we find it hard to understand why to use words (or mantras) in sanscrit or other languages we don't understand. It seems a bit stupid. Well this is the reason: these are words we use because they are magic phrases, they're charged with positive energy and power, they've been charged with people's wishes for thousands of years. The more popular a mantra is, the more power it has gained, and that's how they can help us so effectively. Also by the same token by using a mantra or a strand of beads (or any object) when you pray you charge it, and after a while that mantra and that object will be "charged".
It's very well explained in this extract from Lama Rinpoche's commentary:
I love these books, it's worth ordering if you're interested in buddhism, it's based on a Tibetan tradition but Lama Gangchen Rinpoche does his own thing, and he is extraordinarily smart and clear in his explanations.
I've always felt a very strong instinct for the healing power of sound, from a very young age I knew that by singing I could change things, and this effect is more real, and more physical than we realise. When you vibrate with the universe, that is really something. Singing is not just for recreation, for me mainly it's because I have to.