At my company, email is by far the largest online revenue channel besides organic search. Email marketing software is absolutely critical for customer relationship management.

Without these tools, we have no idea whether someone is opening our emails or not. If they're not opening several of our past emails, we remove them from our list. It's good for them, and for us, as we don't want Gmail or Hotmail flagging us as a spam domain.

We want our email list to consist of qualified leads, not just random people who signed up for a white paper and will likely never actually buy anything from us.

By removing non-engaged subscribers, we can then create customer segments to do things like A/B test different offers. Or set up a one-time offer to only a segment of the email list that hasn't previously engaged with us.