Back in the 1930s, the Jewish Agency was the closest thing the Jews had to a government - which means, not close at all. It operated in a context of international hostility towards its people that is basically unimaginable today, for any group worldwide. Yes, there are some groups today that face seemingly intractable hurdles on their way towards national political expression - the Tibetans spring to mind - but none of them have international hostility remotely resembling the world-wide antisemitism of the 1930s, backed up by the malign indifference of the most powerful governments in the world.
Still, the leaders of the Jewish agency, with David Ben Gurion at their head, plodded on and did their best. Sometimes successfully, oftimes not; sometimes wisely, oftimes not.
After they succeeded, and the State of Israel was founded, the Jewish Agency still had important auxiliary tasks, and again, it did them about as reasonably as any institution peopled by people might be expected to do: With more success than failure, overall.
That was then. Today, it's hard to know what the Jewish Agency is for, what it does, why it still exists even in its much-reduced form - unless perhaps as a vehicle for political machinations. And even that isn't enough to justify its continued existence; we've got more viable vehicles.