Pre-approved

The benefits available at each level can be easily understood when viewed from the seller's perspective. Imagine you're a seller in receipt of multiple offers to purchase your property. A complete stranger (buyer) is asking you to take your property off the market for at least the next two to three weeks while they apply for a loan. As the seller, lets consider the type of buyer you'd prefer to deal with.
Neither pre-qualified nor pre-approved
This buyer provides no evidence that they can afford to purchase your property. You may wonder how serious they are since they're not at least pre-qualified.
Pre-qualified
This buyer has met with a mortgage broker (or lender) and discussed their situation. The buyer has informed the broker regarding their income, expenses, assets and liabilities. The broker may also have seen their credit report. The buyer provided you with a letter from the broker stating an opinion of what the buyer can afford.
Pre-approved
This buyer has provided a broker written evidence of income, expenses, assets, liabilities and credit. All information has been verified by a lender. As a result, much of the paperwork for this buyer's loan has been completed. This buyer will probably be able to close quickly. They provide you with a letter (pre-approval certificate) from the lender. You're as certain as possible that this buyer can close.

As a potential buyer, you can see that being pre-approved will give you the best chance of getting your offer accepted. This is critical in a competitive situation.