Strategic ambiguity. It's very unlikely that NATO will send combat troops to Ukraine. But suggesting that it's a possibility, means that Russia has to prepare for that possibility, which has a cost for the Russians. They can't focus too much on only defeating the Ukrainians, they have to hold something back just in case NATO decides to get involved.

This whole thing is incredibly stupid.

Even if by some miracle Russia 'won' in Ukraine, it'd still be a pyrrhic victory. As the Americans learnt in Iraq and Afghanistan, the occupation is what's difficult, the actual war should have been a breeze. Even the first bit wasn't a breeze for the Russians, Russia won't be able to afford holding on to Ukraine. It will cost them too much money and too many troops. The inevitable insurgency will grind them down.

And thanks to the war, NATO has expanded and is drastically increasing spending. Russia has drastically increased military spending to win the war in Ukraine, but they can't keep up with NATO. It'll be like the Soviet Union all over again, they'll end up spending far more than their economic base allows.