The Fleet, despite what our allies will tell you, is dominated by humans. As one alien diplomat put it, "Terrans do more than their share of the dying for our polity." Like it or not the Fleet evolved mainly from human military services and coffee was a staple of those services.
Coffee has a long history in space. NASA astronaut Don Pettit is credited with creating the first microgravity coffee cup in 2008 (old calendar). With carefully tapered sides and a spout the liquid was held in the cup by surface tension and whisked towards the lips by capillary effect. A few years later the primitive mini fabricators on the International Space Station were set up to print more refined models. They were very expensive, several hundred credits but regarded as worth the cost because you just didn't drink coffee out of a bulb or squeeze bag. Even today most ships carry several of these mugs in case the artificial gravity gives out (sure they can print them easily enough but those few minutes interruption of the supply can be critical, after all, you already lost your grav-con.)
Some aliens are stereotyped as scientists or warriors or pirates (they know who they are!) Humans are often portrayed in alien media with a mug clutched firmly in their hand. Coffee is something of a mystery to them because it makes nearly every species we've encountered ill to various degrees due to the theobromine it contains. The fact that Terrans consume a poisonous drink to maintain an edge has not hurt our image as tough guys on the galactic stage. At least one offworlder scientist studied the coffee plant as a mind controling symbiont. He later disproved the theory. Barely.
Unfortunately the best coffee still comes from Terra. Offworld crops are potable but are characterized as lacking something of the rich taste due to different growing conditions. Coffees raised in space colonies under carefully controlled conditions is a major export. Fleet research continues to seek new better ways to preserve the beans for long cruises, reserving freeze dried rations for emergencies (like losing grav-con).
On most ships one crewman is responsible for ensuring the command crew has their coffee (and tea too, the Fleet does not discriminate). This is in addition to their other duties and usually regarded as a high honor and the holder given some minor concessions for expertise like first crack at the really good stuff. Different ships have different preferred blends, L-5 Kona and Syrtis-Sumatra blend being the most popular. Most galleys stock several different blends (but only generic cheese).
In the last war a logistical hiccup resulted in a coffee shortage on the front lines in the last few weeks. After action analysis indicated Fleet combat efficiency fell by 20% but the human contingents won 40% more engagements. As one analyst put it the Fleet, "Got mean. Really mean." Another (nonhuman) officer suggested coffee be withheld prior to engagements. This was answered by another (human) officer with, "Are you insane?!"