Understanding Voltage/Power line arrows in complex schematic
You answered your own question here:  


  I would understand if this arrow would point out of the schematic to
  show that it is used somewhere else


If you look at the schematic, you can see where it is also used in that snippet, along with some others too:



It is likely there is a backup 12V supply, or the arrow could mean nothing, and it is just there to name the net, so they don't have lines crossing all over the place. It depends who drew it. 

Without the rest of the schematic, it is difficult to say for certain, but what we can say is this DC_IN net is used in other places of the schematic, so naming the net makes the drawing much easier to read.