Human beings breathe 15 times per minute and 21600 times in 24 hours. If you subtract 12 hours of sleeping time at night, we are left with 10800 breaths for a day. Dedicated devotees have firm desire to recite the name of Maharaj for every breath he takes during day hours, but this is difficult for most of them. If we recite mantra according to what is stated in scripture, we would be benefited 100 times for every Shree Swaminarayan Mantra. For rolling 108 beads of Mala we get benefit of 108 * 100 = 10800 for rolling one Mala. This is equivalent to worshiping at every breath. This is the reason for keeping 108 beads in a Mala.
How to roll Mala?
You might have seen a side view of the Murti of Shreeji Maharaj wherein, He rolls the Mala in his right hand. Similarly, you should hold a Mala on the second phalange of the middle finger of the right hand and use the thumb to hold the bead ensuring that the index finger does not touch the Mala. With each movement of bead, we should recite Swaminarayan mahamantra which is Swaminarayan. When we complete rolling of a Mala, we have recited 108 mantras. We would easily know when we have completed 108 counts on Mala then we will touch the big bead. The larger bead is regarded as the "Mer" named after the divine mount Meru. According to the scriptures the solar system revolves around mount meru. After touching, it is essential not to skip the bead and continue by reversing it again. Mala is used as a counter and is considered holy. It should be always kept in a proper place so that unworthy articles may not touch it. For this reason it is kept always in a L shaped bag called Gaumukhi.
Why we do Mala?
Mala inspires devotees to remember Shreeji Maharaj at least once a day especially when we remain busy with worldly affairs. It inspires us to roll Mala one, five or eleven times while worshiping Shreeji Maharaj. We derive satisfaction and confidence by doing so. The introduction of it was felt for chanting of mantra. As a result, devotees roll Mala and for easy counting the mantra, it came into existence. It is ideal for counting the recitation of mantra.