Here are the three reasons why (quotes):
1. To align IT services with the current and future needs of the business and it Customers - internal and external
2. To improve the quality of the IT service delivered.
3. To reduce the long-term cost of service provision.
I am about half half way into study ITIL and I am finding it a hard task not because of the material but because of the length.
What area are you interested in?