The close is the price funds are measured against, so anyone tracking a benchmark wants to trade at exactly that price. That concentrates enormous volume into one print, commonly a tenth or more of a stock's entire daily volume. Imbalance information is published from around 15:50 and moves prices into the bell.
This is where index-inclusion and index-rebalance trades happen, and it is why rebalance days show closing prints far from the last regular-way trade.
Example: a stock trades 8M shares on the day, of which 1.3M print in the closing auction. On a reconstitution Friday the same stock trades 40M shares with 22M in the close alone.
Related: opening-auction, index-rebalance, index-inclusion, market-on-close, reopening-auction