There are a surprising amount of people who have an iPhone as well as a Mac, and they don’t even know that Messages can be synced to your Mac. It’s probably one of the best features of Messages on Mac, it’s significantly easier to respond to messages from your computer, over touch typing on your device.
So, instead of having to type out messages on your phone, you can use your computer when you have it on you, and save yourself a nice amount of time by typing on a real keyboard.
Step 1: Go to your Applications folder, and launch Messages.
Step 2: You know the drill here, open the Messages preferences.
Step 3: Check that your Apple ID is signed in and enabled for the application. Tick off the boxes for the contact points you want to be reached for (the phone number and/or email that is associated with your Apple ID. You can also specify where you want the messages to be sent from when starting a conversation from your computer (for instance, I use my email address as my sending point for my computer, as opposed to my phone number from my iPhone).
All done! Don’t forget, you can adjust your notification settings for Messages in System Preferences, if the pop up banners bother you.