More and more often, cellphones are viewed as a source of potential evidence during criminal investigations. A cellphone can carry a wide variety of data about a person, such as their location data, their text logs, call logs, a history of the websites that they visited, their interactions on social media and much more.
For example, perhaps the police are investigating you for alleged drug offenses. Maybe they think you were trying to buy illegal substances, and they think you used your cellphone to contact potential sellers. The police may be very interested in looking at your location data to see where you have gone or checking your call logs or direct messages to see who you have been in contact with. But to do this, they typically need a search warrant.
The Fourth Amendment
The reason for this is that people in the United States are protected from unreasonable searches and seizures due to the Fourth Amendment. The police can ask for your consent to come in and search your home, for example, but they need a warrant if you do not provide it.
In the modern era, cellphones are viewed the same way. It is very likely that an officer will ask you for your consent to see if you will voluntarily unlock your phone. But if you do not provide consent, they cannot force you to do so. They have to get a warrant, or else it would be an illegal search.
If you do find yourself facing criminal charges, and you believe that the police violated your rights while gathering evidence, it is critical that you understand all of your legal options moving forward.