The Motion Sensor in Phasmophobia can be used to help figure out the movement patterns of a ghost and is required to complete some objectives. The motion sensor can be placed at least halfway up the wall or as low as you want. It cannot be placed on kitchen cupboards, for example. Thankfully, placing it halfway up the wall or above the kitchen counter is certainly fine.
A Motion Sensor does have a frighteningly short range, so having multiple sensors can be very helpful, especially in larger rooms. You can bring up to 4 in the truck. I certainly think it is a good idea to always have the maximum number of these just in case.