These printers work in a very different manner than regular inkjet or laser printers. Where most common printers these days make use of a mechanism where a drum and the paper are charged and a laser is shone onto the drum in the shape of the intended image. The uncharged areas attract the toner which is pulled onto the drum and sticks on the paper to come up with the image that was given to be printed.
Flatbed printers on the other hand make use of UV inks. These inks do not just spray on the material because doing so would just be temporary and the print would fade away in just a couple of days or so. To print on such hard or uneven materials such as wood, fabric, glass, etc. flatbed printers make use of a mechanism that etches if you may, the print on the material. The UV inks made out of monomers are exposed to very a strong UV light source that subsequently polymerizes the ink and embeds the printing image on the material.