The only phone app that doesn't have a security lock in order to access (once already logged in) is the calendar. Assuming the worst and a bad actor steals your device and gets access to the phone, they have unfettered access to the calendar. Piecing data together via the calendar (data aggregation) can quickly compromise a person's identity, their circle of friends/associates, patterns, whereabouts on given days and times, etc. Having a security or biometric lock on the calendar helps add another level of OPSEC.