Firstly, mods not sure if this should be here, feel free to move if in the wrong place

Can someone tell me what the vitamin K injection is for with a newborn?? Is it necessary?? I am unsure about what it is - I know my SIL and BIL didn't get it done with their second and had it on their birthplan - which is why I'm asking cos I'm trying to work a bit more of my birthplan out.