Markets aggregate information in a (usually) efficient way. Information is useful to different people in different ways. Just to take one example, if the stock price of a company falls and stays low for a whole, that could reflect some negative future prospects for the company, which traders know about. That could affect my decision to move across the country to work for that company. That is valuable to me. Since you asked for a source, I’ll give you one: pick up any introductory economics textbook.