Passwords are the most common way a computer uses to authenticate the user. The key point is in the little possibility for any user to guess other users' passwords and then use their identities. Sadly nowadays this often shows to be also the weakest ring of the whole security chain.
Reducing the possibility of password guessing and then improving passwords' strength can greatly improve the overall system security.
crack and cracklib have been written for this purpose: mandate the use of stronger passwords. Whether these passwords are strong enough for his purpose is solely a matter of the user.