Using the main mobile phone is convenient however the phone is usually for most, more likely to get lost, stolen or breaks being carried around. Having an additional mobile number feature allows a spare mobile phone or simply a spare active SIM to be locked away as a backup.
Note, some mobile carriers require an outgoing call or SMS text to be made on a SIM at least once every so many months to prove the SIM number is being used. That's of course down to the user to set-up calendar reminders to use the SIM!
