Cleans up text data before it hits your database and, eventually, your users. The goal is to reduce Cross-Site Scripting (XSS) attacks. Install and forget.
The plugin can figure out which fields it needs to sanitize, or you can specify fields manually. The former is highly recommended. Schemas change.
An added bonus is that you don’t need to sanitize or strip_tags in your views, offering a lil’ performance boost.