Why would a company buy Cognos software on top of Oracle reporting?

I am not technical but am reviewing BI tools, including Cognos, on a data warehousing project for a client that makes extensive use of Oracle. Their main requirement is to enable senior management to carry out complex sales analysis. They are a global FMCG company. My questions are: Why would a company buy software such as Cognos or Business Objects when you already have built-in reporting/ETL functionality within Oracle 9i/10G (OLAP, Discoverer, BI and so on)? Are there any significant limitations with Oracle?
While it is quite true that Oracle has built in a number of reporting facilities, they are not as mature as Cognos or Business Objects (BO). These tools have been built to provide end users with the simplest possible interface possible, while still allowing for very complex analysis. These tools are very robust and their future is very bright.

The questions you need to answer are: What does your organization want to do that they cannot with the Oracle tools? What are your internal limitations? Realize most of these tools do pretty similar things, and you need to define what you need in your environment. I have been working in environments that only required static reporting, and in that case we built reports with SQL*Plus and the users were happy. I have also worked in companies that used Cognos or BO. These companies needed to provide users with more than just static reports and wanted them to have tools that empower them to create and design their own reports.

All I can say here is that each software has its benefits and limitations, but you need to define what you want out of a tool before you decide it's right. Just because you want to drive a Rolls-Royce does not mean that a Lexus won't do what you want. Everyone has different needs, but you need to understand these before deciding to change. Do not change for the sake of change; change to improve your reporting capabilities.