Why should i use MailChimp?
It is hard to keep track of ones customer lists and also when sending thousands of emails it is very important they don't end up in the spam box. It doesn't get better than with MailChimp nor easier.
MailChimp is also free of charge until you start sending loads of e-mails.
Why are newsletters so important?
What makes a webshop strong in the long run is that you keep building your customer lists and stay in touch with them all.
The longer you have been in the business, the more customers you will likely have and these customers combined with some great offers translates into more profit.