I suppose it is not a must , but Iv become so used to the one on the Mate 7 that iv just taken it for granted .
The one on the Mate 7 is really good because it works like a charm and is placed in a very sensible place . On the Mate 7 it allows you to lock apps (like notes etc etc) , it also allows one to unlock the phone by just touching the sensor , no having to first wake the phone or having to press a button etc , just a simple touch and you are at your home screen . It's a small thing ,but something one comes accustomed to especially if you use your phone a lot . There are a couple of other functions , it can operate as a camera shutter and in the future hopefully we will start seeing banking apps etc allowing us to log on etc with a fingerprint scan .
Before the Mate 7 I had the Note 4 and S5 , honestly I never used the scanner on these as I found them to be a bit of a hit or miss . It was just quicker to unlock the phone normally . The Mate 7 ' s scanner just works so well that it becomes second nature to use it when unlocking the device .