Laser printing is an electrostatic digital printing process. It produces high-quality text and graphics (and moderate-quality photographs) by repeatedly passing a laser beam back and forth over a negatively charged cylinder called a "drum" to define a differentially charged image.1 The drum then selectively collects electrically charged powdered ink (toner), and transfers the image to paper, which is then heated in order to permanently fuse the text, imagery, or both. As with digital photocopiers, laser printers employ a xerographic printing process.However, laser printing differs from analog photocopiers in that the image is produced by the direct scanning of the medium across the printer's photoreceptor.This enables laser printing to copy images more quickly than most photocopiers.Źródło: https://en.wikipedia.org/wiki/Laser_printing
Their work is quite hard and I have to admit that you have to really have a fuss to do it.Work after hours, maximum focus and responsibility - definitely not work for everyone.
They have the appropriate knowledge and tools that allow them not only to replace the toners, but also to repair the entire printing device, if such an order will receive from the incoming customer.