It is very important for any guild that participates in guild activities to keep a well-stocked treasury. The treasury is what funds things like unlocking levels for Guild Expedition (GE), building things in Battlegrounds (GBG), and more. 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.
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. 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.
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). Obs can be obtained only by earning the blueprints (BPs) from placing on another’s GB in a reward spot or when the BPs are available from an event (or similar), whereas Atom and Arc BPs can be obtained by any typical method available for earning BPs (so aiding/plundering, placing on a GB reward spot, shrines, etc). Of these three, you will absolutely build Arc because it accelerates everything else, and you’ll want to build Obs in combination. Atom is the only one that really isn’t quite worth the space unless you have a major happiness issue you want to fix permanently. Reason being is that Arc is very useful and Obs is not only a small footprint, but it’s cheap and useful for its size; Atom unfortunately is rather large, and since it’s ME it’s a bit more expensive. Having all three certainly doesn’t hurt, but the two main ones are Arc and Obs.
Most committed players will obtain (likely ahead-of-age) Arc and raise it to at least level 80, which will produce 162 of each good for your age for the treasury on every collection. This by itself is a lot of goods, but the reality of it is that you can only really support GE with this GB alone–adding in GBG puts a big strain on it and so the combo effect of Obs is very important. Bringing Obs up to at least level 60 adds another 108 of every treasury good on collection to what you’ll dump in the guild treasury for your era, giving the treasury a total of 270 of each good from your era every time you collect! If you’re a daily player, then that’s 1,890 of each good from you’re era a week into the treasury. This really adds up! Can you imagine trying to trade for that many goods every week? Insanity!
Why Obs to 60 though, instead of just adding on more Arc levels? While it’s true that you can push your Arc above level 80 (and many long-term players eventually do), it’s far cheaper to raise Obs to level 60 (or greater) than it is to raise Arc the same number of levels it’d require to match to the same amount of goods gained (so to get the extra 108 goods from Arc alone you’d have to add another 54 levels onto Arc to level 134, but with many levels costing well over 10k or 20k FPs that’s truly expensive). Thus, from the perspective of cost, you’re going to get more bang for your FPs from Obs than from Arc when it comes to dumping goods in the treasury once Arc has hit level 80. Eventually Obs will hit a break-even point on cost with Arc, but it’ll take a while. This is due to the fact that Obs is a no-age GB (and thus very cheap to level), whereas Arc is from FE and is far more expensive because of it. The higher the era of a GB, the more expensive it is in general to level.
Many active guilds already require players to have at least one of the treasury GBs, but if you truly value your team being able to make solid efforts in guild activities it would be very wise to plant both an Arc and an Obs and get them well leveled. Start with Arc to level 80 (after you have a stable basic city), stabilize your city to be an advanced player, and then add in Obs to level 60. That will give you a good basis, and you can certainly grow both from there as you’re able.