It saves a bit of time for your embassy if you need a new passport urgently in another country.

> Safeguard Your Documents! Make two copies of all your travel documents in case of emergency. Leave one copy with a trusted friend or relative at home and carry the other separately from your original documents. To help prevent theft, do not carry your passport in your back pocket, and keep it separate from your money.

https://travel.state.gov/content/travel/en/international-tra...