Cynics answer: It keeps the front and rear emergency exits clear so the flight crew can get out faster in an emergency :-)
Real answer: It's all to do with those things you learnt in your physics lessons about levers. Lift is provided by the wings. The further you sit from the wings the greater the downward leverage on them. To compensate for this, extra thrust from the engines is required. This in turn leads to increased fuel consumption. So, to keep fuel consumption to a minimum, Ryanair insist that passengers sit in the seats nearest to the wings.
Chris