Passenger service is usually high-usage enough that the cost of electrification is worthwhile. But many freight-only lines still use diesel, even in areas where a majority of the rail net has been electrified, because little-used supply tracks lack an incentive to electrify them. If we can replace those diesel lines with hydrogen, that's a win.

Also note that the motors themselves are electric, so when this train is running on an electrified section, it could still use the overhead lines. Not sure if modern diesel engines offer that same option (i.e. if the diesel is purely used as a generator for an otherwise electric engine).