Everyone representing themselves is an extreme example but hopefully clearly shows how more representatives are better at representing a group than fewer. Compare a voting result of 50% A, 25% B, 25% C with a system that gives you 1 representative (from A) with a system that gives you 5 representatives (3 from A, 1 from B, 1 from C).
Electoral college permit states to choose electors who can cast their vote in a way that doesn't reflect how people in their state voted for. If a critical amount of states agree they can choose to make the overall result better represent nationwide what all Americans voted for (an election of an election results in a bigger misrepresentation error).