I'm guessing here (I used to study military doctrine a bit, but we're talking 20 years ago, and a lot has changed...).
He's not using them as part of a direct attack. Contamination alone would make that effectively useless.
Instead he'd use them...or more hopefully, the threat of using them...to defend borderlines already established. Take an area via conventional means, and threaten to use that as a staging area for deeper strikes if forces are staging for counterattacks, including transport hubs, airports and the like.
They also represent a pretty strong misdirection play. Let it be known some tac nukes are being released and brought forwards, whilst simultaneously removing conventional forces. That could credibly look like preparation for a strike, whilst also allowing conventional forces to be relieved or relocate elsewhere.
Would Ukraine take advantage of the reduction in conventional forces to advance against the threat of nukes? I mean...maybe, but I doubt it.
Finally, they'd be the ultimate reset button if the war continues to go badly, and Ukranian forces threaten Russian prewar borders. The same countries assisting Ukraine now would basically demand they stand down at that point.