Accession : Spencers, Stewards and Stuarts

A SPENCE or SPENCER is a STEWARD

An INDOOR, or HOUSEHOLD SERVANT

(In contrast to a GROOM or EQUIRY, who attended to HORSES)

By the Mid-17th Century, around the time of The English Civil War, the Mid-point of the Rule of the House of STEWART/STEWARD/SEWARD,

A BUTLER.

SPENCE

The Stewards watched over the throne until it could be reclaimed by a true King of Gondor, an heir of Elendil. When asked by his son Boromir how long a time must pass before a Steward could become a King, if the King did not return, Denethor II replied, "Few years, maybe, in other places of less royalty … In Gondor ten thousand years would not suffice" (reported by Faramir in The Two Towers). 

The Stewards never sat on the throne of the King; instead, they sat on a simple chair of Black Stone placed below it. The symbol of their office was a White Rod. 

The Quenya name for the title is Arandur , "Servant of the King".

Soon after the death of Denethor II, Aragorn II Elessar was crowned King. The Steward Faramir, son of Denethor II, surrendered to the King his Rod of Office, but it was returned to him. 

Faramir nominally ruled briefly as Steward until Elessar's coronation, but since Faramir rested in the Houses of Healing, Prince Imrahil of Dol Amrothtook his place during much of that time. 

Though Faramir became Steward a month and a half before Elessar became King, Faramir is not considered one of the Ruling Stewards.

King Elessar confirmed in Faramir and his descendants the office of Steward of Gondor, and granted him in addition the Princedom of Ithilien, ensuring his line a position as Counsellor of the King [in Hellenistic Greek Terms, the Archon (Who were Advisors to The King)] 

The original model for the Stewards of Gondor is probably the title of steward, a medieval king's follower in Europe. 

In England there arose a powerful office, that of Lord High Steward, which was the principle advisor to the monarch. 

The office has fallen into disuse, but remains to this day for coronations only.

Quark, Son of Keldar, Steward of the House of Quirk

For the answer to your question about Aragorn, see this post which discusses basically why he’s wandering the wild and not ruling a country. But I’ll discuss the Stewards of Gondor here.

The position of steward was created within the first 500 years of the Third Age by King Romendacil I. At first the king could choose anybody to be stewart [sic], and their position was to be both a counselor to the king, and also to rule Gondor if the king was busy elsewhere (i.e. leading the army.) By the year 2000 the position became hereditary.

In the year 2050 King Earnur died without an heir, and his steward Mardil Voronwe. (See more about his death here.) Since the kings fate was unknown (he marched to battle and was never seen again) Mardil vowed to rule in the king’s name until “the king returns.” So, for 27 generations the stewards basically acted as kings, ruling Gondor much the same as any king would. However, they never became kings themselves. This was largely symbolic (for example, they never sat on the throne.)

Faramir once said that Boromir asked his father how long before the steward would become king, and Denethor responded “in Gondor ten thousand years would not suffice.” When Aragorn returned Faramir happily gave him control of Gondor. Aragorn kept Faramir as steward, though, as well as Prince of Ithilien. Faramir’s role would have been more like the stewards before the line of kings ended, as mainly a counselor.