In most modern stock markets there is not tiered access. My order is just as good as anyone else's, and no participants are given unfair notice of another's orders. Firms can spend money and be more competitive in these markets (e.g. better analysts, faster systems, co-location), but anyone has a fair shot at competing. If an order is public, there's nothing wrong with me taking action based on it before you if my only advantage is superior technology.
It seems if you are the broker and also trade for your own company, you would see your clients' orders before they go in. You can send in your order before theirs, thus creating a tiered access.
In any case, most high-frequency operations are only trading for themselves.
Let's say I have a statistical model that is able to identify and exploit statistically significant price deviations. Furthermore, let's say someone has a very similar model. If I'm able to execute my trade before my competitor, I get the profit, and he doesn't. That's one argument. There are many others.