For the founders of future Czechoslovakia, it would be a fatal misstep to not organize something like them. They helped bolster early Czechoslovakism both in the eyes of its domestic supporters and the Entente Powers over at Versailles. Without the legions, I'm not sure whether Czechoslovakia as we know it would come into existence.
There's also the issue of Czechoslovak Legion contributions to the anti-Bolshevik war effort in Russia during the civil war and their smaller (but not insubstantial) contributions to the Western and Italian fron as well.