1. Pre-qualification acts as a dry run of the loan application process. The mortgage lender will use details you provide about your credit, income, assets and debts to arrive at an estimate of how much mortgage you can afford. The whole process may take only minutes or a few hours at most, and is free.
2. While a "pre-qual" is non-binding to the lender (because the information you provide has not been verified), it does serve as a good indication to potential sellers of your general creditworthiness.
3. These days most sellers will NOT accept an offer without at least a pre-approval letter, so if you are serious about buying this is the first step towards getting you in your new home.