GridGain started life as an in-memory data grid. The option to act as an in-memory database means that it is now, technically, an in-memory computing (IMC) platform rather than just a data grid.
The big benefit of using a data grid such as GridGain (or Apache Ignite, of which GridGain is the commercially supported version) is that you can simply add it to your existing environment without any of the pain of replacing your current database. On the other hand, there are times when you positively want to replace existing systems and, of course, there are greenfield developments where you need a new database from the get-go. The optional deployment of GridGain as an in-memory database, as opposed to a pure data grid, offers the flexibility to support all of these scenarios.