It is an estimate, not a commitment. Every new order changes it, and the final cross can land somewhere else entirely because size arrives in the last seconds.
Traders use it as a sentiment reading on the pending cross, and as the reference for deciding whether to add offsetting liquidity against a published imbalance-message.
Example: at 15:45 the indicative is 74.50; at 15:55 it is 74.20; at 15:59:30 a 400,000-share sell order arrives and the cross prints 73.95. Someone who lifted offers at 74.50 assuming the indicative was firm paid 55 cents above the eventual close, which on 10,000 shares is $5,500.
Related: imbalance-message, opening-auction, closing-auction, price-discovery