This is a vast oversimplification, but some markets are ruthless zero sum games, others are not. mature markets that are either fixed in size, growing very little, or shrinking are usually zero sum games with all competitors trying to screw each other and their customers for every cent.
Other markets, especially those with potential for large growth, behave differently. Growing the market is more important than trying to maximize your share of the market. You don't want to have the smallest piece of the pie, but sacrificing a little bit of your share in exchange for a larger pie can be a win.
In knowledge businesses like consulting, educating customers often means educating competitors simultaneously. However, it can still be a win to share knowledge.
Finally, one must consider brand. Sometimes you trade knowledge for brand. If you write about programming, you may make your competitors better programmers. Some clients may feel empowered to write their own code instead of hiring a consultant. But your brand can now open opportunities for you that didn't exist before.
I guess what I'm saying is that knowledge is power, but sharing it can be an investment under certain circumstances.
