Well, next time you are in game, check out the fleet holdings in the fleet menu.
You will find projects there that are used to progress the Fleet base and embassy (future holdings will be added, my personal guess for the next one is a dilithium refinery).
Each project requires different kinds of contributions to be completed. the most important one, as it's exclusively used for fleet projects are fleet marks (also gives you the highest return in fleet credits).
Other contributions are dilithium and depending on the project doffs, commodities, data samples or gear like photon torpedos.
Every contribution gives you fleet credits in return which you need to buy special fleet gear.
We have exclusive vendors on the fleet starbase and embassy that allow you to buy high quality gear, consumables, buffs etc.
You can also get slightly higher quality starships which are not exactly cheap, though.
While they don't need much fleet credits (relatively speaking) you need to have fleet ship modules which you can buy on the c-store or from the exchange, and those are pricey).
Other than that, the more we progress the fleet base/embassy the more it becomes a fully equipped social hub which can at the end service your entire needs you only find on other starbases at the moment.
Right now we are a mid level fleet on the verge of updating our base to T3 (of five tiers) and our first upgrade of the embassy is under way.