Working capital (WC) is an important metric for all businesses, regardless of their size. WC is a signal of a company's operating liquidity . Having enough WC means that the company should be able to pay for all of its short-term expenses and liabilities.
Large companies pay attention to WC for the same reason as small ones do: WC is a measure of liquidity, and thus is a measure of their future credit-worthiness. Companies who want to borrow by issuing bonds or purchasing commercial paper (a market of large, short-term loans for big companies) will find it more expensive if they do not have enough WC. If they are a public company, their stock price may fall if the market doesn't believe they have adequate WC.
For small businesses and start-ups, unable to access financial markets for borrowing, WC has more dire implications. WC can also be described as the amount of money that a small business or start-up needs to stay in operation. Start-ups need to pay attention to their WC because it is the amount of money they need to keep the business running until they break-even (start earning a net profit).
On one hand, WC is important to because it is a measure of a company's ability to pay off short-term expenses or debts. On the other hand, too much working capital means that some assets are not being invested for the long-term, so they are not being put to good use in helping the company grow as much as possible.
WC is only one measure of a company's operating liquidity. It is not the only measure, and it is certainly not a guarantee of a company's ability to pay. A company may have positive WC, but not enough cash to pay an expense tomorrow. Similarly, a company may have negative WC, but may be able to adjust some of their debt into long-term debt in order to reduce their current liabilities.
WC is an important metric, but is not the whole story of a company's financial health.