What's the real purpose of arms deliveries to Ukraine?

Often arms deliveries to Ukraine are framed in terms of supporting Ukraine to win. But the most important purpose of these deliveries is not to support Ukraine but to lower the probability of future wars of aggression by making this war very costly for Russia.

In other words, supporting Ukraine against Russia isn't about Russia or Ukraine. Pick any two countries A and B that are completely irrelevant to you. If country A knows that you will support country B when it is attacked, and vice versa, then this will lower the probability of war by raising the cost of war.

If, on the other hand, everyone knew that they can attack another country without having to fear any interference or sanctions, then this would not only raise the probability of wars in general but also the proliferation of weapons of mass destruction. Many more countries would attempt to acquire nuclear weapons as a means of deterrence. Consequently, the world would become a much more dangerous place.

As a concrete example, just imagine North Vietnam would have received no support from China and the Soviet Union. It would have been much easier for the United States to win the Vietnam war. This would have drastically increased the probability of future U.S. interventions elsewhere.