system
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human
ELI5: Why are there noble ranks? Why wasn't everyone just a "lord," why were there dukes, counts, earls, barons and the like? How did this develop and did the different levels have different powers and responsibilities?
gpt
In the English (later UK) system, the different noble titles had different origins. That is, they weren't part of a unified system from the beginning.

Earl was the oldest title, and dates back to Anglo-Saxon times. The earls were originally basically royal governors of the shires or counties of England. 

Baron was a Norman French title brought in by William the Conqueror.  It literally means "man," as in "the king's man"--this title was used in a generic sense for all nobles holding land from the king, but became specific to those who didn't have some other formal title (some of the English earls accepted William as king and retained their titles, while William appointed replacements for others). Thus it became the lowest title of nobility sort of by default. 

Duke is from Latin *dux*, meaning leader. In the later years of the Roman Empire, *dux* was the military commander for a Roman province. When the Roman Empire collapsed, dukes became local warlords, and later senior regional leaders for kings. The old Roman hierarchy left little impression in England after the Anglo-Saxon invasion, but *dux* was sometimes used as the Latin translation of earl.  William the Conqueror was Duke of Normandy (in France) before he became King of England, and for a long time the Duke of Normandy (as an additional title held by the King of England) was the only duke around in England. The title of duke was introduced to England in the 1300s as a grade of nobility senior to earls. Eventually duke became a title mostly used for members of the royal family, e.g. brothers or sons of the king. 

The titles of marquess and viscount were introduced into the English nobility even later. Viscount originally meant vice-count (count was the equivalent of earl in continental Europe), and was originally a kind of deputy to a count. By the time the title was introduced in England, though, it was just because the king wanted to be able to grant a title more dignified than baron but less dignified than earl. Similarly, marquess (marquis in continental Europe) was introduced in England merely as a title to go between earl and duke, but originally it meant a marcher lord, i.e. the count of a march province. A march province was a province on the frontiers or borders of a kingdom, and a marquis was considered more important than an ordinary count because they held a more strategically important position.