Nobody is obliged to purchase a security certificate for their website. If, for example, you created your own website to show people photos of your pets, you wouldn't need one. However, without using the 'SSL' (secure socket layer) way of transferring data, which is required to get a security certificate, modern browsers would warn visitors to your site that it's 'not secure' (which wouldn't actually matter in the least).
The Answerbank [as an example] doesn't really need to use SSL technology (since all the data you send to it is intended to be on public view anyway) but, in order to prevent people getting worried through seeing 'not secure' messages, uses it anyway. (That's why there's a padlock symbol next to your address bar, showing that this site has a valid security certificate).
If The Answerbank staff forgot to renew their security certificate, this site would still be safe to use but your browser would warn you that the certificate it was seeing was no longer valid. It's not the job of your browser to totally block potentially unsafe sites, as it has no way of knowing which sites are actually safe or unsafe; all it can do is to let you know that there might be a problem. So, after providing you with a warning, it allows you to continue using the site that your post is about but at your own risk.