Thus far (as of Space Age Asteroid Belt) there are three GBs that will donate to the treasury: Observatory (Obs for short, it has no age), Atomimum (Atom for short, it is from Modern Era or ME), and The Arc (Arc for short, it is from The Future or FE). Sometimes manual donations will still be necessary, but having well-leveled treasury GBs in your guild helps to cut down on how much of that is actually needed, and potentially how often. While you can make manual donations to the treasury by making or trading for goods and then depositing them, it is far easier to build and level up a Great Building (GB) that will donate goods on your behalf. Without it many guild activities come to a screeching halt, and it is easier to drain a treasury than it is to stock it by a long shot. The treasury is what funds things like unlocking levels for Guild Expedition (GE), building things in Battlegrounds (GBG), and more.

It is very important for any guild that participates in guild activities to keep a well-stocked treasury.

Forge of empires how does the arc work