While the League of Nations did not prevent WW2, it was not a useless organization. One thing it did that helped many, was the creation of the Nansen passports, which gave stateless Russians a passport issued by the League of Nations, and thus enabled them to escape the Soviet Union in 1922. The reason they became stateless, was because after the Soviet Union won the civil war in 1921, Lenin decided to remove the citizenship of all Russian emigres, people who tried to emigrate from the Soviet Union after the civil war. This removal of citizenship made 800.000 Russians stateless, and thus removed any chance of immigration to another country. The passports issued by the League of Nations to the Russian emigres, worked as a valid passport for stateless people. This meant that they were not stranded in the first country they arrived in after emigrating from Russia, and the Nansen passports thus enabled their immigration to other countries. The reason the passports were called Nansen passports, was because Fridtjof Nansen as High Commissioner for Refugees for the League of Nations, came up with the idea to create the passports. Nansen had also previously been in charge of delivering aid to 2 million Russians in 1921, during the famine in Russia.

The Nansen passport, after its introduction by the League of Nations in 1922, was a success and 52 countries ended up accepting it as a passport. The passport was also extended to people outside Russia, and from 1933 it also covered Armenian, Assyrian and Turkish refugees. Fridtjof Nansen also received the Nobel Peace Prize in 1922 for his work to help refugees.

What is "useless"? The League of Nations was very useful in terms of things like what the person above has talked about, in terms of aid and humanitarian efforts (I believe there was a health organisation which had it's roots in the LoN)

But in terms of stopping violence, it was effectively toothless. Like the UN, it had stages of warding off violence with trade embargoes ramping up to military involvement. The problem was, I believe, without the USA as a member, it could not embargo a country effectively. There can be debate as to how far this made them toothless (after all, many countries were in effective embargo due to protectionist policies from other nations) but I would argue it compromised the weight with which they could come down on a nation acting out.

Besides what has already been mentioned, in my experience the League of Nations is mostly remembered within academic discourse for it's legacy as a very important hallmark in the history of intergovernmental institutions. Despite it's flaws, it was an innovative concept.