Sometimes you are in a context when handling the mail is not effective, possible or reasonable. You may need to wait for a certain time, be in a particular place, or have certain resources or pre-requisites. For those emails, you can defer the handling. BlueMail has ready made time frames set for you: later today, next day/week/month, or someday in the future.
Note: that if you use the Incoming Filter, you won’t see emails marked as Later+.