Ukraine depends on weapons and ammunition shipments from the EU and US. Continued US support continues to be uncertain. The EU is set on continued support of Ukraine, but an attack on Finland or the Baltic countries might be a way to force EU countries to reroute many of these shipments to defending that new flank

The other angle is that Putin is in a precarious situation if he can't win in Ukraine. Too much has been invested, to many sacrifices made. And a lot of his political power depends on him looking strong. He could be looking for some way to find a quick victory somewhere else

I don't believe either of these are likely scenarios. But Putin is increasingly backed into the corner and has already proven that he will take risky bets on starting new wars. Every country bordering Russia is well-advised to make themselves look like a well-defended place where the Russian military would only be bogged down in another quagmire