Dis, let's say there is s seat where the Tories win on 20,005 Lab getss 20,000 and the Libs get 19,995, there is only one mp's seat up for grabs in that constituency so the Tory wins and the Labour and Lib Dem votes don't count. It is a straight win/lose situation in each constituency.
Under a pr system, you would look to see who the definite losers were. Then you'd check their ballot papers for a 'second preference' and then count those votes again, giving them to the Tories and Labour as suits until you have an overall verdict.
So you end up with a better (though not perfect) representation of people's voting intentions.