In Russia Catholicism isn’t really a thing.

In Ukraine, it has a stronger institutional presence than the number of followers suggests: the Ukrainian Catholic University is an important institution.

In Poland, Romania, and Germany, it’s very important- far beyond what the number of adherents would suggest.

In countries far removed from the conflict, Argentina, say, it has immense weight. Far beyond what the number of adherents would suggest.

Also many of the bishops (Francis included) have practical, hands on experience operating in countries at war and actual war zones. They are also extremely savvy with the press and civil society groups.

When your country is at war, the Pope is a very nice friend to have.

I recently talked to a priest who spent decades in Latin America during its many wars. He would be a very good friend for the Ukrainians to have.

I'd guess the #1 motive is trying to stay relevant as an authority on moral and moral-adjacent issues. Then #2 is to alert the faithful (and trying-hard posers) to the official Catholic position on the subject. Changing many minds in Russia is probably far, far down the Vatican's "If Only" list.