More often than any other document I have, but not often.
Non-travel uses, I most recently needed it 4 years ago when I got my Real ID Driver's License.
Before that, in 2005 when I switched jobs to provide citizenship/ right to work in the U.S.
Passports were a huge help for my kids when they were kids. They are physically a more durable document, unlike a birth certificate. We used Passports for proof of age when they signed up for Little League and Soccer and enrolled in school and got jobs.
And when my daughter got her Driver's License, I took both her birth certificate and Passport and the DMV clerk wanted the Passport, which they are allowed to accept at face value as proof of your age and citizenship. A birth certificate , there is a greater risk of it being counterfeit. At DMV three different people that need to examine the document to certify it is legit. The Passport bypassed that process.