The passwords don't have to
The passwords don't have to be complex. You can use something as simple as "password" for your password. The system won't reject it. It justs lets you know if yours is weak or strong.
As to why passwords are used at all? There are lots of reasons. Here's just some:
We have all sorts of data on this site that needs to be protected from hackers, spammers, and spam bots, such as your email address and any info you may add to your profile, address, city, state, etc.
Logins and passwords also tell a system that you are who you say you are. You don't want someone to log in as you and post a bunch of stuff that you wouldn't say. When a site doesn't require a password, anyone who has your screen name can do just that.
Not only that, by giving the site your login and password, it tells the system what access you are allowed to have. If we didn't require passwords, you could simply use my account name and have access to all my webmaster access. You would be able to hose up this entire site. That would be a no-no.
So, there are a bunch of reasons, many I haven't covered here, as to why it is very important to have passwords.
Hope this helps. Welcome!