Just think about all those water softening plants, water tanks and water cranes that BR ripped out after steam was abolished. If this takes off, NR will have to reinstate some!
Its a trial, proof of concept, whatever you will. Better to try a full scale testbed in an existing loco and suss out the problems, rather than spend even more money on a completely new vehicle. And its better than building a fleet of the things and then finding it won't work.
However, even a failure will be an interesting failure!