If you will be crossing the US border, you may want to protect your phone. As this Lifehacker article notes, by law if you have touch or facial recognition unlocking on your phone, you can be required to unlock it, but you cannot legally be forced to enter a passcode or swipe pattern – agents must have reasonable suspicion that you have done something criminal. And… both Android and iOS have handy ways to go into a lockdown mode that disables touch/face unlocking.
Why would you care if Homeland Security looks at your phone? Well, because the Fourth Amendment protects us from unreasonable searches. The data on your phone is yours. If your phone is searched, agents can download copies of everything on it, including all of your contacts, your calendar, your photos, your email, and all the business information of your employer. It’s basically a giant detailed profile of your entire life. That should not be seized by the government without cause.
In other words, it’s an American principle and it’s worth holding to it.
Note that the Android lockdown mode requires Android Pie. If your phone doesn’t have Pie yet, you have the wrong phone, and you should get yourself a Pixel, Essential, or OnePlus.