Now suppose there are three parties, X, Y and Z. People do vote for Z, but they never win anything. They are always clearly last.
Seat A is: X 400 votes, Y 310 votes and Z 290 votes.
Seat B is: X 350 votes, Y 360 votes and Z 290 votes.
Seat C is: X 350 votes, Y 360 votes and Z 290 votes.
X wins A, Y wins B and C so Y has 2 seats, X has 1 and Z has 0 under the current system. It’s closer, because Z has grabbed some votes, but the same result.
X still gets the largest number of votes overall.
With proportional representation, Z could say, well, we got roughly one third of all the votes, so we should have one seat, so even though they were last every time, they could still get just as many seats as X or Y ie one each.
It’s a bit simplified, but roughly right.