Let’s take positive results of a phase 3 study of a biotech company as an example.
The news is still not published. The insider buys shares. This additional demand drives up the share price relative to where the stock would otherwise have traded, until the news is released and the stock price jumps to the level it would have reached anyway without insider trading.
Results:
1) The insider gains.
2) Stockholders who sell their shares to the insider gains, because (in all likelihood) they would have sold anyway before the news was released. Now they receive a higher price due to the added demand of the insider.
3) Stockholders who would have bought shares instead of the insider but now does not, because they are outbid by the insider, loses.
Now, consider the results above. Who do you think should rightfully benefit from the rising share price?
In a perfect world without a time lag between the time the study analysis is completed and the time the results are made publicly available, the stockholders who held the shares at this particular time would receive the full gain. I am sure we would all agree that this is fair and right.
In a non-perfect world with a time lag, the result of the insider trading is in fact to push up the price, so the shareholders who we just agreed were the rightful owners of the gain, receive a greater part of the gain, than they would without insider trading.
Based on this you could argue insider trading is a benefit for society.
P.S. Significant insider "trading" goes undetected by default. When insiders refrain from trading due to inside information, where they would otherwise have traded, they can make enormous gains. This, of course, is impossible to prove and thus "a perfect crime."