First of all, what is an email drip campaign? It is often known as drip marketing, automated email campaign, lifecycle emails and autoresponders all having the same sense.
In email drip campaign there is a set of marketing emails that will be shoot out automatically as scheduled. The first email will go out once someone signs up, second will be sent on the next scheduled date, followed by the successive emails as per automated setup. Preferentially, the emails can be differed based on triggers, or activities performed by the person like signing up for the service or making a purchase, which is why they're also called behavioural emails.
Drip campaigns, as presented above, are automated sets of emails that are triggered on particular timelines or as per user behaviour. It enables you to communicate with groups of people based on activities such as signs up for an account or how often the user visits your site. Every time a drip email is triggered, it is released from a queue of pre-written emails—there's no need to write manually and mail particularly. These emails can be personalized by the receiver's name, contacts, company info, and much more.
Keeping it simple, drip marketing is a concern of giving people the appropriate information on the right time. If somebody just subscribed to the blog newsletter, for example, a welcome email can be sent right away using a drip campaign, followed by the scheduled emails that show off some of your most-read content.
The importance of drip emails is that this all occurs automatically based on triggers and user segments that are defined by you.