Hi !
Very good question and unfortunately, most BI Managers/Architects do not ask this question and simply go on using Cognos as a purely tabular data tool and then blame Cognos for not being capable of delivering any better results than what they get from it. This is because they have no idea of how to create a Data Mart using star schema design which is required for using Cognos as a BI tool. Without it, when someone connects Cognos to a database which is design for transactional purposes (like an ERP system) then the only thing they can get it to pull data extracts from different tables which do not have relation to the other one thereby limiting the ability of Cognos in mainly two ways:
1> Limited reports can be made
2> Every report will take lot more time to make then it should.
The purpose of Cognos is to be used as a BI tool and that means making simple as well as complicated reports quickly and easily by the end users who do not have any idea of the complexities of the database. The objective is to present a front end which is non-technical. I will be happy to share a video of a product which will show you the front end and how complicated reports and visualizations are made in seconds as opposed to days. Send me a PM with your email and I will send it to you.