Because a Seth Godin say, it is "permission marketing". Inbox is customer's personal online space. Anything stepping in there, needs a prior permission from the customer. Therefore, if you manage to grab his attention, get his email, then you should pay due respect to his “personal” space. How? By keeping your emails and offerings in sync with his personality.
So do you understand now the reason 'personalization' is such a buzzword these days? It gets you things done, your product sold, distraction-free. Still, with the overwhelming amount of data and statistics, it gets complicated to devise conversion-focused email marketing campaigns. This infographic below summarizes the 9 proven (tested successful) ways in which any marketer can deliver a more personalized experience to his subscribers.
Infographic by: targetingmantra