Because shit happens, and you don't have always a monitor, an usb keyboard, the hmdi adapter, or whatever the fuck fails that day :-)
Place wpa_supplicant.conf in sdcard root folder. NOTE THAT THIS ONLY works in the first launch. You'll need to format again sdcard with those changes included.
Create an empty file named ssh
in sdcard root too to enable service.
SSH default port is 22
username: pi
password: raspberry