Company executives and directors - i.e. insiders - know their business more intimately than any Wall Street analyst ever would. They know when their products are better than those of their competitors, when margin is improving. They understand the dynamics of the industry they are in and their competitive advantage. Last but not least, they would most certainly refrain from buying their own stock if they knew about any legal, technological or environmental risk.
In 1976, Peter Lynch, head of Fidelity Magellan Fund, said "Insiders might sell their shares for any number of reasons, but they buy them for only one: they think the price will rise."