They try to make a point by demonstrating what is NATO expansion to them. Their only goal is to reduce NATO presence, not to expand anywhere.
By creating a menace without actual invasion they try to discredit US government which makes failing predictions one after other and shifting public opinion, so that NATO will be forced to negotiate a number of treaties on favorable conditions. After all, NATO did seem to promise non-expansion as recently found archive document shows.
Destabilizing “the west” and preventing Ukraine from joining NATO. Look at the pandemic… Soon there will be toil about defensive measurements and expenses. The economy wont like a chronic war threat either. Putin just has to move Russia’s troops every now and then.
Russia splits the West, which is already in some chaos.
Germany is heavily reliant on Russian gas and has been unenthusiastic about sanctions, never mind stronger action. France is being "diplomatic". The UK is an incoherent mess because of Brexit and a mentally ill prime minister.
Also, energy prices. Which happen to benefit both sides.
It's exactly the strategy Russian trolls farms use on social media. Find multiple wedge issues, weaponise them - typically with narcissistic gaslighting and DARVO - inflame grudges and bad feelings on both sides, try to destroy the other side's common identity and purpose.
Putin will have to declare a victory of some sort, but it's likely to be formal annexation of a fairly small region. He'll also be able to blame poverty in Russia on Western sanctions.
The real weapon is the constant psychological and emotional pressure and deliberate confusion.