The closing line is the last price available before an event starts, after the market has absorbed all news and money. It is generally the sharpest probability estimate in existence. Closing line value means having taken a better price earlier: you backed 2.20 and it closed at 2.00. Do that consistently and you almost certainly hold a real edge, because you are repeatedly out-predicting the market's final verdict.
CLV matters because it separates skill from luck far faster than profit does. Results over a few dozen bets are noise; consistently beating the close over the same sample is signal. It is also why prices move at all — early value gets bet away — and why acting quickly on flagged value matters.
You take 2.20; the line closes 2.00. Implied probability moved from 45.5% to 50%. If the close is accurate, your bet carried EV = (0.50 × 2.20) − 1 = +10% — banked at the moment you beat the move, whatever the result.