Device security has become much more important to us as we add more and more confidential and personal information to our smartphones and tablets. As a result we've seen a significant increase in the number of devices sporting fingerprint sensors.
It's just that some OEMs are putting them in the wrong place.
Apple and Samsung have it right, the sensor should be right on the front of the device where it can be reached whether your phone is flat on the desk or in your hand. The alternate solutions, favoured by Google, Huawei and others, to put the sensor on the back, or the bizarre side mounted sensor on the Xperia Z5, both compromise unlocking the phone in some common scenarios.
Primarily, with the phone flat on your desk anything but a front mounted fingerprint sensor is inaccessible. Which means if you want to interact with it you have to pick the phone up.
I should also say that the same is true of the Iris scanner built into the Lumia 950 and HP Elite x3. Unless your phone is at an angle or in a dock you've got to pick it up to unlock, or indulge in some impromptu yoga to get your eyes over the scanner.
So, take note OEMs, fingerprint scanner on the front of your devices in future please.