Why doesn't CodeIgniter use native PHP sessions?

Question

PHP Framework "CodeIgniter" provides a session class that does not use native PHP sessions. Instead, they developed their own session system using cookies.

On their CodeIgniter.com website, they write the following:

Note: The Session class does not utilize native PHP sessions. It generates its own session data, offering more flexibility for developers. 

As mentioned below, only a maximum of 4 KB can be saved.

 Note: Cookies can only hold 4KB of data, so be careful not to exceed the capacity. The encryption process in particular produces a longer data string than the original so keep careful track of how much data you are storing. 

So how does this give the programmer more flexibility? What was the actual reason developers didn't use native PHP sessions?

Thank you for the information!