Preapprovals become relevant to sellers at the offer stage. So if you don’t have preapproval, many sellers won’t even consider your offer.
If a seller does accept your offer without preapproval, you’ll need to apply for the mortgage. If you’re denied, you’ll lose the due diligence you submitted with your offer.
If you go through the preapproval process before submitting an offer and get denied, ask your lender why. They can help guide you to the next steps you should take to make your financial situation more appealing, allowing you to eventually qualify.