When you receive a notification on the iPhone--say, for a text message--you don't have to unlock your phone to respond to it. You can actually respond to all sorts of notifications from the lock screen, no unlocking needed.
Not only can you respond to text messages, but you can also respond to reminders, events, Facebook messages, and more. Basically, if it pops up on your lock screen, you can probably deal with it from there.
In a lot of cases, such as with Slack notifications, when you swipe left, you will only have the option to remove the notification.
In many instances however, you'll have more options. Here we've received a text message.
If we swipe left, we have the option to reply with one tap, and we don't need to open iMessage. We can also dismiss the message, which means it will remain unread in iMessage.
If we reply, then the keyboard will pop up and we can tap out a quick message to the sender without unlocking the phone first.
You can also use these swiping skills on reminders. Here, we're given options to dismiss the reminder (which doesn't make it go away), mark it as complete, or snooze it.
If you use Facebook Messenger, you have several options as well, aside from the usual dismiss and reply, you can also "Mute" talkative friends. Keep in mind however, it will stay muted until you go back in to Messenger and unmute it.
You can also interact directly with calendar notifications. The only options available are to dismiss it or snooze it. Nevertheless, if you do have an event that you want to be reminded of at a later time, snooze is definitely a useful option.
The point is: for every app that posts a notification on the lock screen, there are probably options to interact with it. Often, you may only be able to dismiss it, but as you've seen from the examples in this article, there are often other options available--and you don't have to usually unlock your phone and start the app to tap out a quick reply or snooze a reminder. Next time you get a notification, give it a swipe and see what you can do.