Valve added a horde mode called Mann vs Machine to Team Fortress 2 in August of 2012 where you fight waves of computer controlled robots and with that update, came botkiller weapons. These came in strange quality and were purely cosmetic variants of regular strange weapons. At the time of writing this post, there are six different types of botkillers: rust, blood, silver, gold, carbonado and diamond.
Personally, I don’t see how gold, carbonado and diamond robot heads make any sense because every robot you fight in MvM is silver, but given TF2’s incredibly forgiving realm of reality, anything is possible. Rust, and blood bot heads make complete sense to me and here’s why:
A soldier and demoman may have “shed blood” fighting on the front lines and so their spoils of war include blood covered robot heads hanging from weapons.
A sniper (or any other class character) could pick up an old robot head semi buried in the ground and put it on his sniper rifle. A rust bot head makes the most sense here. Silver also makes sense but rust models look better in my humble opinion. Snipers play from far away so blood botkillers don’t make sense.
A spy’s knife is small and can’t possibly be attached to a large heavy robot head (which would make them too heavy to use) so a small head is used on the model and you could really go with anything here. Even gold, carbonado and diamond mini robot heads make sense on a knife.