A General is one thing, a Field-Marshal is quite another.
A General is a Very Important Dude. (I met one, once. General Rockingham actually noticed that I lived. I am still in a state of shock.)
A Field-Marshal, essentially, has his own WAR. He commands an entire segment of the War Effort and the Generals scurry to carry out HIS orders.
In North Africa in WW2, the Field Marshal ran the entire campaign. EVERYTHING and EVERYONE reported to him. Same in NW Europe: Monty was THE Man.
To HAVE a Field Marshal you have to have a War in the first place. It is exclusively a Wartime rank; when the Peace is settled, the Field-Marshals generally revert back to Lieutenant-General. A Lieutenant-General is an officer of General's rank who is a LIEUTENANT (helper, assistant, gopher) to the Field-Marshal, who has been appointed by the Monarch to run the Monarch's War.
Remember, our rank structure is different from the American. We start with a Brigadier, who runs a brigade. We then go to a Major-General, who is a BIG General ("major" meaning "important" from Latin); he usually runs a division. Next up is straight General: the guy who can run anything.... and generally runs a CORPS. And only then do we get to the Field-Marshal's helper: the Lieutenant-General. If he is in an independent command, he will usually run an ARMY. Field-Marshals can run GROUPS of ARMIES if they have to. The precise scope of the rank is determined by the duties/ responsibilities at hand and is at the pleasure of the reigning Monarch. A Field-Marshal can be replaced should the war situation require. They can even tell the RSM what to do!!!!!!!!
Robertson was the FIRST man to make the entire climb. To date, no-one has followed in his footsteps. Robertson retired in 1920, serving through the entire War in one capacity or another.
The American equivalent is General of the Army (5 stars arranged in a circle): a 5-star General in an Army in which 4 stars normally is tops. The name Eisenhower comes to mind.
The Germans had their own idea of a Field-Marshal, made it a regular rank. The result was that you could have half a dozen in a single room. We never had more than ONE for an entire campaign.
Hope this helps.