They could have already fulfilled their main purpose on many occasions. As long as they exist, they continue to fulfill it. They preserve knowledge and experience. Beings on the level of First Ones are probably able to access it, complementing their already large baggage of information.
But at the time of the Great Burn, the Rangers were not even close to First Ones. Only a thousand years had passed since the end of the Shadow Wars.
The Vindrizi would have continued carrying knowledge. Developing an ability to tap into it would have taken even the most advanced younger races another few thousand years.
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"We are the universe, trying to figure itself out.
Unfortunately we as software lack any coherent documentation."
-- Delenn