A NASA scientist was recently forced to give up the PIN to his work phone.
Your privacy rights are significantly reduced when crossing national borders. This can become an issue if you want to bring a phone or laptop that you use for business. You might have legal or contractual requirements to keep the information on your devices confidential.
Doctors could have patient phone numbers stored on the phone, either explicitly in the address book or implicitly as a list of recent phone-calls. Even if you can't see them, they are likely still there.
Lawyers could have attorney-client privileged documents, emails, etc.
Engineers could have intellectual property.
US Customs or ICE can force you to surrender your device(s) for up to 30 days for a forensic analysis. They do not need any probable cause or suspicious. What they may NOT do is force you to reveal any PIN numbers and passwords without a court order. Unfortunately, if you choose to exercise your right to remain silent, you may be detained long enough to miss your flight, barred from entering the country, or otherwise severely inconvenienced. The rules in other nations vary, and in some countries, you can be forced to surrender both your mobile device and your passwords.
The best way to protect sensitive data is not to physically bring it with you across national borders. That way you cannot be forced to give up your passcodes or decryption keys.