The Level 2 data is simply showing the depth of the market. If I am trading shares with my broker I have the option of viewing only the top 10 bid/ask prices in the depth or all of the data (which sometimes can be a very long list). With another broker I get the top ten bid and ask prices and how many orders are available for each price level, or I have the option of listing each order separately for each price level (in order of when the order was placed). I get the same kind of data if trading options. I do not know about futures because I don't trade them.
Simply this data may be important to a trader because it may give an indication of whether there are more buyers or sellers in the market, which in turn may (but not always) give an indication of which way the market may be moving.
As an example the price depth below shows WBC before market open with sellers outweighing the buyers in both numbers and volume.
This gives an indication that prices may drop when the market opens. Of course there could be some good news coming out prior to market open or just after, causing a flood of buyers into the market and sellers to cancel their orders. This would change everything around with more buyers than sellers and indicate that prices may now be going up.
The market depth is an important aspect to look at before putting an order in, as it can give an indication of which way the market is moving, especially in a very liquid security or market.