Using the smartphone as a computer mouse
With an accelerometer the smartphone can detect how much it is being moved. It can therefore used as a computer mouse. The communication can be either USB or BlueTooth.
The mouse will not require anything to rest on, so it will work in free air. If used in free air, it will probably be useful to have one of the buttons to have the function "ignore input" - just like when you lift a normal mouse from the table.
When doing a presentation you can move more freely and are not required to be close to the computer.
If you travel with your laptop and you prefer using a mouse instead of the builtin pointing device, you will not have to have a bring a mouse in your travel bag: You can use your smartphone that you are probably bringing anyway.
With a 3D accelerometer the smartphone can even be used as a 3D mouse. Again an "ignore input" button is probably needed.