I would like to tell your first how does a mobile vibrator works?
There is one DC motor inside the mobile phone with an eccentric weight on it's shaft. When we put our mobile phone in vibration mode, and we receive any call to our mobile then motor starts and produces vibration in order to inform us about a coming call.
Why does mobile vibrator is required?
In some area either ringtone is not allowed or you have to switch off your mobile. If even in that area you need to switch on your mobile phone but don't want to disturb others then put your mobile in vibration mode. When some call will come on your mobile phone it will notify you by vibrating the mobile but won't disturb others. So, company has made it to have as per requirements of customers.
That's why mobile do have vibrator.