to bolster its defense against NATO, Russia's largest threat which is present in Eastern Europe, these countries are not non-aligned. Taking important strategic positions gives Russia leverage, reduces defense burden and ensures access to important geographic access points (like to the Black Sea). It increases Russia's opportunities and margin for error and diminishes their opponents'.
It's kind of a weird question honestly, why do you think India, China and the United States pursue territorial interests and fight wars? History and conventional warfare hasn't ended just because countries have nukes.
> to bolster its defense against NATO, Russia's largest threat
The idea of NATO attacking Russia is bonkers. They're barely moving to defend Ukraine. "NATO is a threat" is post hoc rationalization for Russia's own revanchist behavior; even the Russian propagandists who push it don't take it seriously.
Of course NATO is a threat, or at least it is credibly perceived as one in Moscow. Do you think the US would tolerate the Warsaw pact on its own borders merely because it has nuclear weapons? (Cuba knows the answer to this question, the US does not).
There's plenty of examples like the deployment of missile defenses to Eastern Europe (which contrary to the name is strategically an aggressive move, as they're first strike enablers, diminishing strategic deterrents. The one thing Russia is supposed to rely on). The basic point is, no country on earth has stopped deploying its military in the pursuit of national security merely because it is a nuclear power.
> Do you think the US would tolerate the Warsaw pact on its own borders
NATO is a defensive treaty organization, joined by consent. Warsaw Pact was the result of USSR using military force to subjugate its neighbors. There's no comparison, and I see no reason to engage such tripe further.
There are also stable and peaceful borders, and Putin could absolutely have chosen to have one with Europe. Instead he chose instability in order to expand by force to areas that Russia used to control.
The only reason for US military presence in Europe is the Russian threat. The US would gladly move forces to the Pacific and Europe would be happy to have a peaceful and democratic Russia on their border and to see the US go.
Putin chose empire-building by force. He could have chosen peace and economic prosperity instead. Just compare the development of Russia and the former Warsaw pact countries.