A dividend is one mechanism for financial rigor and focus, since shareholders don't like dividends being cut. Having large piles of cash lying around is a temptation to use it for sub-optimal returns, or even worse, empire-building.
Another reason is that the shareholder functions more like an owner. Without a dividend the managers have a less clear responsibility to the shareholders. Maybe they act wisely, maybe not, but there is no real constraints on their behavior. The stock then acts more like a lottery ticket than owning a business, because you don't get anything out until you sell. Whereas, if the company pays you something because you own shares and therefore you own a piece of the profits, the ownership structure and the management structures line up better.
Another reason is that the shareholder functions more like an owner. Without a dividend the managers have a less clear responsibility to the shareholders. Maybe they act wisely, maybe not, but there is no real constraints on their behavior. The stock then acts more like a lottery ticket than owning a business, because you don't get anything out until you sell. Whereas, if the company pays you something because you own shares and therefore you own a piece of the profits, the ownership structure and the management structures line up better.