The European plan developed in the wake of the Eya… etc eruption in 2011 is based on airlines conducting Safety Risk Assessments and regulators accepting these SRAs. The idea is that flight plans filed by airlines with SRAs will be automatically accepted by the authorities in the national states overflown.
[disclaimer: what follows is from memory, it may contain some inaccuracies]
The ash cloud dispersal model distinguishes between medium concentrations of <4mmg/m2 and higher concentrations above 4. Most airlines have SRAs for <4 but noone as far as I am aware has had one approved for >4.
When this procedure was tested in May with the Grim… etc eruption, some states followed the new approach and never closed their airspace – except of course since nobody had an SRA for >4, nobody filed flightplans for >4 airspace (not quite true but almost). Some countries announced that they would close their airspace at >4 but follow the SRA model for <4.
The one exception was Germany, which (a) announced that it would only approve SRAs (for German airlines) at <2, and (b) insisted that non-German airlines would need German SRA approval to fly in German airspace. Consequently, for practical purposes this meant that while the rest of Europe was operating to a <4 threshold, Germany was at <2. It also meant that German airlines could not fly in <4 airspace over other countries because they did not have an SRA approved by their own CAA.
So, as to Easyjet’s motive, I suspect that it may have to do with obtaining CAA approval for an SRA for >4. If that were the case, they would be automatically allowed to fly through >4 airspace over those countries which had fully adopted the new procedure – airspace through which no other airlines (unless they had obtained their own SRA>4 approval) would be able to fly.
Obviously, the data gathered by this device during the next eruption will be invaluable in measuring the ash cloud dispersal and drawing more precisely the contours for medium and high concentrations – to the benefit of everybody.