I have done a fair bit of meditation using mantras. The idea of this type of meditation, as with many forms, is to synchronize body, speech and mind. The turning of the beads on the mala, as well as bringing the body to stillness work with the first. The repetition of the mantra, which can be any series of syllables (or a single one) works with speech. The mind is generally focussed on some type of visualization at the same time. Sometimes one simply visualizes the words of the mantra spinning in a circle.
All of this provides three points of mindfulness and awareness, which is very grounding, and eventually helps to slow down and gain perspective on the many many thoughts and feelings that flip through our minds all the time. As with any activity that completely absorbs our attention, there can also be lovely moments of peace, where mundane concerns disappear from view and the mind relaxes.