The lower spreads mean lower costs for investors, because Nasdaq investors generally do not trade directly with one another. Instead, they usually buy and sell from market-makers, brokerage firms that flip shares between buyers and sellers and keep the spread for themselves.
Bigger spreads mean bigger gaps between what buyers pay and sellers receive. For example, a spread of 10 cents a share means that the buyer pays $100 more for 1,000 shares than the seller receives.