You are correct in the reasoning for people putting higher/lower orders in dutchie, but please remember if I was to put an order in for say AMP at $50 for 1 share, I would not buy at $50, but I would buy at the opening price for the day.
As for there been volume before the market opens, 3 things cause this. One can be an off market transfer of shares (these are usual very large orders), the second and most common is the exercising of options, and the 3rd is early morning trading mostly from the US.
The market depth has a long history stretching back to 1800. A lot of traders used to use the tape to predict where markets where going and thus how the market depth go so popular for people that had access to it back then (namely at the exchanges). In today’s market it means nothing as it can change in a few seconds, very little can be sourced from it.
Cheers