Roboberry, over at Raspberry Connect, has documented a way to automatically create a wifi hotspot on your Pi if your wifi connection to your router fails, or if there is no router to connect to. This is a great idea because it means you can connect to your Pi ‘on the road’ without worrying about needing to pre-set wifi credentials. Take a look here if this is what you’ve always wanted! Be warned! It’s extensive, and technical, but it has recently been updated to work with Raspbian Stretch. Obviously, this is ideal for Pi 3 or Pi Zero W owners, but will also work with compatible dongles on other models.