Aer Lingus got some of their registrations with the airports they serve. I believe seeing EI
also, but I think this was returned.
As far as why they did this, probably just to be unique. I personally think this is pretty cool.
I know BA
did a similar thing with their RR
powered 777's. They started with G-YMMA, to signify the new millenium. Y= year MM
= 2000 in the old roman numerals, and the A was the first in the fleet, and it went on from there.
My personal point of view, if a special reg is available, they why not take it. It's kinda like personal plates on a car, only for your aircraft.
Break ground, and head into the wind. Don't break wind and head into the ground.