If there were to be a system where there were no constituencies but all the votes were to be added up nationally, then everybody's vote would have some affect on the result of the election, but nobody's vote would change the result of the election.
In the UK when people vote they just vote for their preferred party. But when the results are added up nationally, the results don't match the vote. For example, in the 2010 election David cameron got 36.1% of the vote of the nation, but 47% of the seats in the House of Commons. This is just not fair.