Lancia to planet sized ships like the Zhikkhen and Helionix, motherships provide transport, defence, attack, and more. All three races have some form of mothership, but only the Ta'Elans and the Axizians use them for more than just transportation. The humans use motherships mainly for transporting other ships over long distances of space, and they are essentially large, mobile space stations. The Ta'Elans and Axizians use motherships within the same roles as smaller ships.
You can out more about each race's motherships here: