Why a strong password is needed?
Simple answer: to prevent being hack (easy password can be hack using bruteforce method). For example http://0fajarpurnama0.weebly.com/blog/simple-password-bruteforce-demonstration-using-hydra.
Elaborated answer: to prevent unintended access to your accounts because:
In my opinion a strong password:
For additional super strong password:
Example of strong password: My8.oP.
Example of super strong password: ╥F0r.m3-w4T1ng<f0R>YoU╝
Although you have a strong password, it does not protect you from social engineering. For example, clicking on a suspicious link, opening a suspicious attachment, running a suspicious programming, entering your password on a fake login page because you didn't double check the link, etc. This is because you are giving your password to them, in other words, they don't need to guess.