Why does the Ultimaker 3D Printer has a Heater + Heater transfer plate (aluminium) + Glass?
I wonder why a glass plate, and if is possible to remove the glass and print directly in the aluminium plate adjusting the heating.
Printing directly onto aluminum is something I've never seen before, likely due to the fact that PLA (and other materials) do not adhere reliably to aluminum. Instead, many opt to use blue tape, kapton tape, PEI, buildtak/commercial build surface, or an additional build surface, such as glass. When heated, clean glass can be directly printed on. The use of a glue stick, wood glue, isopropyl alcohol, the above adhesion aids, and others can help adhere your part better hot or cold.
Can you remove the glass, add any of the above to the aluminum plate, and print on that? So long as it's a clean, flat surface, yes. But it'll be more work for you to replace or clean the build surface, as you won't be able to simply remove the glass and replace it. You're not gaining much by taking out the glass. A slightly faster bed heat-up, perhaps.
As for why Ultimaker went with an aluminum transfer plate, that is a slightly more engineering oriented question.