The ability to target our customers so well that we can create custom messages specific only towards them (and not other similar people).
It's also easy enough use within an email marketing tool like Mailchimp or Constant Contact as it integrates very nicely into their platforms too! We've been able to send targeted emails at scale without having any technical knowledge required - something which was previously difficult when sending via traditional methods such as print ads etc., where each campaign would require separate campaigns set up using different software/methods if even possible before being sent out!!