why to use doctrine with codeIgniter

[eluser]mysterious[/eluser]
I see a lot of tutorials that teaches users how to use doctrine with CI. I am confused why do developers will like to use doctrine with CI. What are advantages of using doctrine with codeIgniter? What kind of projects may require doctrine? Anything else that you want to share about ci+doctrine??

[eluser]OliverHR[/eluser]
http://www.symfony-project.org/gentle-in...r-Doctrine
http://en.wikipedia.org/wiki/Object-relational_mapping