The accumulator is probably nitrogen charged with enough PSI to assure that it won't be maxed out under the heaviest load capability of the FEL. I believe the accumulator would be an additional cushion to that of the front tires. The links that Kenny posted above have some good information.
Back in the late 70's/early 80's I ran a Deere 7720 combine with a wide header and often in terraced fields. Had to be real careful to not gouge the header into the ground as it would pick up dirt and could cause damage. I noticed one day that there was an accumulator on the combine that was teed into the header lift cylinders ports. Got to reading in the manual about the purpose of the accumulator so opened the valve to make it effective. What it did is provide a level of flotation so that if one end of the header contacted the ground it could easily float over the terrace without digging in.
It did take some getting used to, however, as the response to asking the header to raise was sluggish so required planning ahead. It eventually became "second nature" and worked as intended.