Professional money managers and speculators in general, at least in principle, more efficiently allocate capital as a result of their speculative activities. This causes overall returns to increase. In a world without money managers/other speculators, market returns would be lower overall.
The simple model: in a world without money managers/speculators, overall returns might be 2%. In a world with 1 speculator, returns might be 2.1% and that speculator might achieve returns of 4%. In a world with many speculators, they might all achieve returns of 4%, and none of them would be beating the market. They are creating value, however.
(Another reason they don't beat the market is that many of them are not trying to. The manager of my Vanguard Target Retirement 2050 fund is currently trying to beat the market. In 2040 his goal will be to minimize risk.)
Bond traders and stock traders alike both balance the risk and reward of a company's offering in order to determine a fair value for that offering. Valuing risk and reward is valuing the company and valuing the company is the most critical part of a stock offering. Any post on here about raising money from a VC will talk about the valuation of the startup.
PS: Consider a company with consistent revenue/dividends but zero transitions on the stock market. Its value is going to be heavily influenced by stock market conditions, but it's ability to raise money is going to be dominated by the state of the bond market.
Edit: My point is what it means when a transaction between Invester A and Invester B happens on the stock market at a given price.
http://en.wikipedia.org/wiki/Trade-off_theory_of_capital_str...