It's related to a productivity strategy called "inbox zero" where you immediately deal with every email in your inbox when you check your mail. "Deal with" might be to delete it if it's junk, respond immediately if you can, or defer it to later. A lot of these clients are aimed at making this process as fast and efficient as possible. So taking actions on an email are often just a swipe, and there are facilities to defer an email to a particular time, or adding it to a to-do list.
It's mainly intended at people who get too much email.