I wear it because I am supposed to wear it. It is an ancient Vaishnava tradition that appears in the Caitanya-caritamrta. Interestingly, japa-mala is not in the CC. Chanting beads may be the result of European influence, when the Portuguese Catholics came to India. That may be why it is not mentioned earlier. According to Wikipedia: “The first Portuguese encounter with the subcontinent was on 20 May 1498 when Vasco da Gama reached Calicut on Malabar Coast.” So da Gama came to India when Lord Caitanya was about two years old. Whether or not people 500 years ago used japa beads is not a philosophical point, it is an external detail that we can discuss. It is not offensive to inquire into history, since it does not involve any philosophical or spiritual issue.