* Fulfils his old goal of expanding Russia to old soviet borders (see Chechnya, Georgia, Crimea etc).
* Distracts from internal problems with an external enemy (a tried and true tactic of dictators)
* Provides a buffer from Nato and Europe (long a preoccupation)
The bet here is that the West will do nothing if he takes Ukraine. Given the current messaging from the US and Germany (Embassy evacuations, disposition of fleet, Biden slipup Fri, 5000 troops), he's probably right.
You don't move this many troops and ships etc as a feint (nor would you need to), the die is already cast, and the only uncertainty is what the West will do about it IMO - at present it looks like not much, as they didn't do much about Crimea.
Worst case for Putin is a long war which bleeds the country of money and resources, so he'll want it over quickly and a puppet ruler installed, at which point he can withdraw and plan to slowly absorb the Ukraine or leave it as a satellite state like Belarus.