Which is the scenario where you can have replication for SQL server over Clustering ? other than on line or offline availability of db?

 I think you can consider here replication in your ecommerce scenario because once you do replication and have your db on DR site could resolve like high availability issue  whis is the actual issue ,if your DB is down then what you will do so replication and clustering type of technologies comes into everybody's mind.

If your reports are taking time so for that you can consider database indexing etc technologies to enhance the speed.

Clustering is solely for disaster scenario if your main db server is down so passive node could come and play as active node as of my knowledge. You can do any administration activities on passive node.

You can have replication of SQL Server over clustering if you are running an E-Commerce website, and have huge no of data and customers. While accesing the Admin section, the database will be having huge load during downloading report for longer interval. This database load can make the feel on the customer to have speed issue in website front end. You can use cluster method for this, so that all the admin operations will be running on clusterd DB, and the customer doesnt have any issues of the website speed, while admin operations will also be smoother