One use case not being discussed here is that by using these types of devices, you can power a device/sensor out in the field without a really expensive installation. You can run these off of a motor housing, compressor discharge, steam line, etc. It is about being able to provide power where it is very costly to bring.
To install a sensor/instrument in a plant,refinery,etc., you have to set up scaffolding and/or use a cherry picker, run cable (usually armored) through cable trays, bend and install conduit, have all of the right junction boxes, glands, etc. You've already spent 10's of thousands just to get power out to your device.
Solar is not really an option in some places. Either you've got no sunlight because you're in a pipe rack and you've got to route a cable from a PV panel (go back to installation cost) or you're in a hazardous rated area of the plant where you have explosive atmospheres and need special Ex rated equipment.