These markers were a little bonus for those of us who backed the Spectre Miniatures Kick Starter in 2014. Personally I think it was a little thing the Spectre Miniatures guys quickly cobbled together when they realised that they had far more success that they had originally planned for, but I like them and so I am not complaining.
As far as I know they were exclusive to the KS deal, but it wouldn't be too hard to recreate- just a tarpaulin made from Green Stuff, with a load of weapons and packs pushed into it. I think I might give it a go myself, as I have enough bits and bobs from the Warlord Games WW2 plastic sprue and the Wargames Factory's plastic zombie survivors.
In game terms they can represent ammo dumps, spawning points or objectives, which should come in handy.