In the previous guides we connected the ESP8266 to a local network using hard coded credentials. It is fine for messing around with examples and when developing sketches but not very convenient or practicle for final projects.
What happens if you want to move the ESP8266 to another network or if you buy a new router? You need to change the sketch and re-upload. It would be better if we could pick the network to use at run time. This is exactly what WifiManager allows.