It's a show of force.

A lot of leftists in west have accused Russia of being a paper bear and for barking without bite. Last night they showed that they're not afraid to escalate this conflict if the situation requires it.

As for the how: most likely they produced a couple of these platforms and simply didn't use a payload.

Or they also have non-nuclear variants of these missile and have decided to start using it.

At the end of the day these are Russians. Just like Trump, they are very unpredictable.