Although ExpressionEngine makes it easy for the HTML-illiterate to update a website, that alone would not be worth the $99.95+ price tag.
ExpressionEngine not only allows us to separate out the code, but makes it easy to implement very advanced features, which are more often associated with large social-networking websites, like Facebook or MySpace. Visitors can leave comments, or join your site as fully-fledged members. You can set up RSS feeds, a photo gallery, a mailing list, a calendar, a wiki, or a discussion forum, and you can sell items through PayPal.
ExpressionEngine also comes packed with industry-leading features to keep your website secure and spam-free amid all the activity. You can force people to register as members before they can submit comments to your site; you can personally review every member application before approving; or you can allow non-members to submit data if they type in a captcha correctly. (A captcha is an image of letters that a person must correctly type in order to submit data; it ensures that it is not an automated spambot that is submitting the comment or member application.)
Further, you can prevent people from submitting the same comment twice, restrict how much data a person can request from your website in a certain timeframe (to prevent denial of service attacks), blacklist certain words so that they cannot be used in comments, and blacklist URLs or IP addresses of spammers that you don't want accessing your site.