Tin and Cadmium are both used as platings to reduce corrosion on the spring-steel based metals used in tube sockets. Tin works in the same way it does on tin cans, by excluding oxygen from making contact with the steel. Cadmium, being a more chemically "active" metal, behaves like zinc in that it not only excludes oxygen but acts as a sacrificial layer, oxidizing to protect the steel should corrosion actually begin.
Both are also easier to solder than steel.
Cadmium has been found to be toxic. It's a carcinogen, and workers are cautioned about handling it. In fact the control levers underneath some fine old turntables were plated with cadmium for corrosion prevention; those working on them should take precautions such as using gloves and dust masks. I learned this only after about forty years of owning and using such a turntable. There's no exposure if you just use such a component, but if you service it, you can easily come into contact with the cadmium plating, which is soft and can come off as a powder if it is abraded.
Incidentally the dull silvery plating on the Dynaco Mark II is reportedly cadmium; for some reason it seems harder and less easily abraded than that on the turntables I spoke of.