Your Wi-Fi network doesn't end at your office walls. The network extends well beyond your office in the air. If your wireless access point is not properly secured, other people can gain access to it.
It is wise to create a long random password or passphrase. It can make it much more difficult for intruders to breach your network.
If you have a weak password, there is always a possibility that it can be found from word lists. Word lists that contain thousands of passwords can be freely downloaded from the internet.
The link to the video below shows a simple penetration test, which demonstrates the importance of strong passwords.