It is well known that users tend to use simple passwords while registration in any site or webservice. But this is of great danger because lists of common passwords are well known to hacker community and are used actively.
So, it is of great respect for any site/webservice to check passwords for presence in these lists and to warn user about this and do not allow to register (sign up) with such (compromised!) password. If your website doesn't have a captcha on login form it takes only few seconds to test all 10K most popular passwords and find out that used by the user of your site.
So, be aware! Please use this list of 10K most popular passwords while user sign up and don't allow users to use these compromised passwords!
The list of passwords is taken from Discourse Repo: http://github.com/discourse