What's the difference between entering your PIN, and signing for a debit card purchase? An Investing in America article has the answer:

Then there's the next generation debit card--similar to a credit card--you sign for purchases. It can be swiped by a credit card reader and then go through the credit card authorizing network. The money for these purchases may not come out of your account immediately, as it does when you use your pin number. There's typically a day or two of lag time.

In fact, the article mentions that your bank may charge you a small fee for PIN transactions, while signed transactions are free.