If you know what you are talking about, it is still in your interest to let everyone else know after you have purchased shares so that your shares will rise faster and higher.

Just because you are confident that something will go in a certain direction without telling everyone, doesn't mean you won't benefit from doing so.

As many economists like to say, markets can remain irrational longer than you can remain solvent.

Your saying really only applies to trading algorithms.

if you think company X is going to do well, you invest in them, and then comment about how amazing it is. (That might drive up the price even more).