The idea is that such missiles have far greater range than current missile tech and that allows them to take routes that were previously impossible. Such missiles can completely bypass US defences and detection. This is a first strike weapon, and it's a great illustration of the danger of abandoning nuclear treaties policy that US pursued.

Probably because the US is spending every last dollar it has to surround Russia with military bases, so Russia feels like it needs to continue to escalate its nuclear first strike capabilities.