Anyone still wondering whether the financial lives of consumers will be aggregated on-line—that is, brought together and managed in one place—hasn’t looked at the numbers. Publicly available data suggest that US households could save an average of $530 a year if they managed their saving and borrowing more skillfully. Purchasing financial services from the most competitive provider would save a further $950. And paying bills when they are due—neither unnecessarily early nor late—would save $180 (Exhibit 1).

The McKinsey study that supplied these figures painted a picture of consumers holding fat checking-account balances that earned no interest at all while racking up sizable credit card debts at hair-raising interest rates. Ordinary savings and certificate-of-deposit accounts are still popular, even though they earn far less than government bond funds having little risk and comparable liquidity. The average consumer’s cash flow, brimming with overdue receivables and nonfinanceable payables that are settled too quickly, is truly abysmal.

Of course, consumers were able to integrate their finances before the coming of the Internet. But since doing so typically involved paying a financial adviser to manage many different accounts and accruing transaction costs each time money was moved among them, the cost was prohibitive...