Relational Database Search using Google Search Appliance

[sdani]

One of the differences between Google Mini and Google Search Appliance is that the search appliance provide the capability to Index databases directly.

[google.com...]

i.e. one can create full text indexes on relational databases, which are external to the database, sitting on a different physical box.

What is the use case of such an application? Why would anyone want to Index the content of the relational database external to the database?

Thanks