The WiFI and nRF24L01+ present two different ways to communicate:
The WiFi can communicate with the internet, an IP based network, and other WiFi modules. With this you could of course make a mesh network and get great range.
The nRF module can communicate with other nRF modules like it using either direct or mesh libraries - these are simpler transceivers that are cheap ($4 at digistump) and work with Arduinos and just about any other board. These are short range and the intention is that you can use them to have other devices talk to the DigiX and the DigiX act as a gateway to the internet for those devices - so perhaps some cheap $10-$15 minimalist Arduino compatibles with nRFs throughout a house talking to a DigiX, which puts all of the data from those nodes online.
I'm sure many won't use the nRF, and no doubt the WiFi is the bigger draw - but I wanted to create a device that could allow the use of many cheaper devices rather than making it necessary to buy many of the same expensive device when creating a network of nodes.