Events centuries past is what we call history :) When a nation has outlived the centuries creating a vast cultural background, people tend to care about its importance. [1], [2], [3]

To answer your question straight, the reason the return of the Parthenon marbles matters, is morality. [4]

One cannot appreciate the significance of such creations to the fullest when they are taken out of context.

[1] http://www.parthenonuk.com/ [2] http://www.parthenoninternational.org/ [3] http://www.marblesreunited.org.uk/ [4] http://www.forbes.com/sites/realspin/2014/12/23/the-british-...