When the market was booming preapproval was virtually required. When there are multiple offers coming in to the seller, not having preapproval would get your offer ignored.
What was worthless was prequalified. This process just asked for you to provide numbers, absolutely no checking was done, and no credit check was done.
Preapproval does have some value for the buyer because they require you to gather all the required documents: pay stubs, bank statements, IRS forms. Just getting those documents in place now, will prevent you from wasting time in the future. If you are honest in everything you provide, it will mean that you will have a good understanding of what you can afford. The lender will be able to accurately calculate the monthly payment you can afford, and that via the interest rate can translate into the maximum price you can pay.
The preapproval letter should not mention the maximum loan you can get, because that weakens your bargaining position.
Preapproval does speed the process of final approval because they have already been through the process with you. They will be double checking their calculations, and will be looking for changes. Did you buy a car since the preapproval? Did somebody switch jobs?
They might even do one last check a few days before settlement.