NikkiV - I think that your minutes/SIM card expire after 30 days or so. If you "top off" by buying some minutes before the expiry period, you get to keep your SIM active. The main reason for wanting to keep your SIM active is that you get to keep the same phone number and don't have to stop at a store to get a new SIM each time you arrive to Jamaica on subsequent trips.

It's new to me, I'm not speaking from experience, so those who know for sure, please correct me if I'm mistaken!