The best sort of business you can generally get is repeat business for several reasons. First, your marketing costs to existing customers can be very low which means that the profit margins you make on repeat orders tend to be higher than first time orders.
Second, existing customers are more likely to buy from you than new customers because they have already trusted you once and know that you deliver. So in general you can expect higher conversion rates from campaigns aimed at existing customers than you would from campaigns aimed at acquiring new customers.
Many businesses recognise the value and likelihood of repeat business when they calculate the returns they need from new customer acquisition. They may even be comfortable losing money to acquire a new customer if they know that, on average, new customers go on to place future orders with them that will more than cover the original acquisition costs.
Online there are many techniques you can use to get repeat orders from your customers. A couple of interesting techniques involve remarketing and the use of autoresponders.