You decide to create your own party. The Chocolate Party.
Under FPTP, after intense campaigning, you get 15% of the votes nationally. Millions of people voted for the Chocolate Partly. Sadly, you were crushed in every race. So you don’t get any representative in parliament. You are powerless. The millions of people who vote for you ? They don’t matter.
Under proportional representation, 15 % of the votes means you get 15% of the seats in Parliament.