There are different reasons why companies outsource but more or less, some of these reasons are similar.

In the article, In Depth: Customers Analyze Outsourcing Vendors And Strategies, it points out the top 10 reasons why companies outsource.

Reasons for Outsourcing:
1. Cost-savings - 65%
2. Flexibility in increasing/decreasing IT capacity - 41%
3. Knowledge transfer and industry-specific expertise - 34%
4. reliability -28%
5. Removal of fixed costs - 26%
6. Operational expertise - 26%
7. Better support for internal users - 22%
8. Lack of available in-house personnel - 21%
9. Speed to market - 20%
10. Improved IT performance - 20%