What are the skeleton head keys that you earn used for?

They're for your actual missions inside of your campain like for example im on level 497 but my mission level with the skeloton keys is 220 and you can only aquire those keys in the snowfall selection or whatever type of season it is either way it will be on your left and heres a lil tip i wish someone woulda told me a long ass time ago but if you start playin that selection there is 3 different stages ok so if you play all 3 then you have to wait till the next day to start it all over again unless u refresh it with 75 diamonds but me personaly i get almost all the way till the end of stage 3 then hit the pause button to end it or die on purpose either way dont finish the 3rd stage n then itll start u off in the begining then u can play non stop like that n earn more change n plus it starts off with a different gun so if u dont like the one itbstarts u off with do it till u like the gun it does give u oh n if u get the peircing ammo perk that will give you unlimmited ammo only for auto weopons n not shotguns hope anybody reads this cuz this is some damn good advice so use it to ur advantage im still tryna find out how to get the HK7 so if anyone knows how please lemme know happy killin