Current customers cost you much less to keep happy, than new ones take to acquire. You can spend a vast amount, and end up offering a lot of incentives, in order to attract those who don’t yet know you. With your existing customers, the hard work is already done. By looking after your customers you not only secure repeat business, but create your own brand ambassadors.
The aim is to make them feel special, to thank them, reward their repeat business and encourage their word of mouth (it is as powerful online as it is off).