Two reasons:
Dispersion of assets makes them more secure. Most of the Russian land based missiles are mobile (unless you believe they have operational hypersonics) because they like moving them around so much. Our accuracy on our independently targeted and manuevering multiple warhead missiles is now less than a yard and that makes them really nervous.
Second reason is political in several ways. First if you're Poland or one of the Baltics it's disconcerting when the missiles are on your border (3 minute flight times versus 6 mins if they're in Russia. It's why Russia has always hated the Turks for letting us first base ICBMs there and later Pershings.
Another political reason is that when an ally let's you put nukes there, and you trust them enough to put the nukes there, it demonstrates a "special relationship." After the Kazakstan fiasco, Putin wants somebody that looks like they're standing side by side with Russia. He's realizing he has gotten killed in the PR war.
And I would be flabbergasted if Putin used tactical nukes. Even the Chinese would go crazy because they would see that as making it easier for us to use tactical nukes to defend Taiwan. Not to mention they are downwind of Ukraine.