A preapproval is an actual, completed loan package that has been submitted to an underwriter for approval. The only piece that would be missing is the identified property.
With a preapproval, once the property has be identified, the only documents that would need to be provied would be an appraisal and a contract.
The preapproved loan can close in as little as 2 weeks.
A preapproved demonstrates to the seller that you will be able to close your transaction subject to appraisal (or not if you waive appraisal contingency).
Check out my blog Preapproval v. Prequalification