Users want to ensure their data does not fall into the wrong hands if another user has access to their device.
The system allows the user to set up a PIN or similar password to lock down access to part or all of the app and to protect the data accordingly.
Maintaining data privacy is a multifaceted challenge. It involves both accounting for and reducing unnecessary but legitimate communication (such as capturing and reselling data to third parties), as well as preventing illegitimate extraction from casual bad-faith actors or illegal intrusions from willful criminals. The downside to increasing security is the extra barriers to entry that may see a decrease in adoption or use. In this case, an app which is otherwise not password protected may escalate the security around one part of its functionality when that data is especially sensitive.