Hey suttree, your "Risks" straw for WBC (Westpac) is a little strange. Firstly, what company are you looking at? - because it's certainly not Westpac, which trades at over $25/share, whereas you are showing the bids/offers spread for a company that would likely open at around 31 to 32c based on those numbers. The pre-open market auction serves a couple of purposes:
The second point is why people often bid at prices that they know are higher than where the opening price will settle, i.e. just to ensure that their trade is among the first to be processed. In the example you gave, you'll see there's an "indicative price" of $0.315 shown above the Market Depth column. That tells you where the price will open based on the bids and offers shown. That opening price can obviously change as more bids and offers are added, or existing bids and offers are ammended. If those bids and offers were to stay exactly like that, that company would open with a share price of $0.315 and ALL bids (buys) above that price would have been processed at $0.315 and all offers (sells) below that price would also be processed at $0.315. However it shows that there is surplus volume of 115,232 shares at that price point, so placing a pre-market order at that indicative price would NOT guarantee that your order would be processed at the open. It might, but that depends where in the pecking order it ends up. If people are keen to buy, they will often place an order at a higher buy price pre-open, knowing that they are going to get the opening price. Conversely, people keen to sell may lodge a sell order at a low price point just to ensure their order is processed at the opening price. The orders that are processed first are the lowest priced sell orders and the highest priced buy orders, but they all get processed at the same price - the opening price, not at the prices that the buyers and sellers have nominated. Once the market opens, that all changes, but that's how it works in the pre-market auction.
The same sort of thing happens after the market closes at 4pm Sydney time when people position themselves for the closing auction. With high liquidity companies (that trade a lot) it is often in the closing auction that significant volume is traded as people look to position themselves or else close out a trade, which is why the share prices of many companies will often move a fair bit in the closing auction.
If you think about it, the same thing applies here on Strawman.com. If you're keen to buy a company, you can set a very high buy price, safe in the knowledge that your order will be processed at the closing price today as long as the closing price is equal to or below your limit price.