There are some things that CI doesn't do. CI was intended to be a small and lightweight framework. The zipped download for version 1.7.2 is only 2.1 MB and is downloaded in seconds, whereas the Zend framework is 10 MB. It won't answer all the problems you will have. But it does:
Make it easy and quick to program in PHP
Structure your site and help you through the architectural decisions
As a result of being lightweight, it does not have as many features as some of its rivals. Other frameworks such as Rails, CakePHP, or Symfony have scaffolding and generators. These tools automatically write certain basic scripts for you.
Once you have set up a database, Rails creates out-of-the-box web pages to do basic Create, Read, Update, and Delete (CRUD) operations on the database tables. In addition, Rails allows you to write generators—pieces of code that automatically write other basic scripts. The Rails community has created quite a lot of these, so you can automatically generate scripts that do all sorts of clever things.
CI concentrates on making basic things easy. Some of the things it handles are:
Sounds familiar? All of these are basic processes, which you will have to go through if you're building a dynamic website. CI makes these processes easier, and makes your code more likely to work. Join this outstanding community; it will help you in case you need it (at the CodeIgniter forums). You can even share code at the Wiki (http://codeigniter.com/wiki/), and you will find why CI is so popular.