If it"s got a Schrader valve on it, and is tied in with the hydraulics, it"s got to be an accumulator. Typically they are put on hydraulic systems to buffer shocks to the system. For instance if the machine has an aux connection and was being used with a hydraulic hammer, someone may have added it to help eliminate the surge/shock on the system caused by the hammer. Usually the hammer itself will have one on it, but who knows.
Along the same lines, if your traveling over a long distance with a load, it will also help take some of the shock out caused by the machine and load bouncing. I know several different brands used to use the same setup on their loaders in years past. Usually they were designed with a valve so you could put the accumulator into the system when traveling, and take it out when doing things like loading a truck, etc.
Beyond that I don"t know why anyone would put one in a system, as they are designed to do what I just described, or to accumulate oil, just as their name implies. In that case, basically what it does, is to increase the volume of the system, in essence storing more oil under pressure, to provide a surge of oil greater than the system itself can provide, to perform some function. They are using a setup like this right now on some big trucks doing stop and go driving (like garbage trucks) where the oil is compressed as the truck is running and brakes, and then releasing it to a hydraulic motor to help get the truck back up to speed when it pulls off, in an effort to reduce the load on the main diesel engine.