To expand on the abusive relationship aspect, say your partner, parents, etc. want to keep you from contacting people, maybe your family/friends, maybe the police, maybe your doctor (for example to seek abortion services, or maybe treatment for a health issue they’ve been trying to withhold medication from you for) they may check your phone to see who you’ve been contacting or even take your phone from you to prevent you from using it.
There’s also of course less savory reasons, cheating, dealing drugs, or other illegal activities.
And somewhat less likely, undercover officers, FBI agents, investigative journalists, etc. whose cover could potentially be compromised if the wrong people are around and that second phone goes off.