Why did kings and cities try to legislate who could wear what, and did it work?

To keep rank, morals and trade in order, at least on paper. Sumptuary laws were meant to regulate and reinforce social hierarchies and morals through restrictions on clothing, food and luxury spending, often depending on a person's rank. They were also used to regulate the balance of trade by limiting the market for expensive imported goods. In Late Medieval cities they let the nobility limit the conspicuous consumption of the prosperous bourgeoisie.

The rules could be strict and strange. In the early Roman Empire men were forbidden to wear silk, deemed effeminate and morally suspect, and at the height of the Empire infringing the rules on Tyrian purple was treason punishable by death. England's Caps Act of 1571, meant to stimulate domestic wool, required all males over six, except the nobility, to wear wool caps on Sundays and holidays on pain of a fine.

Did they work? Mostly not. Historian Lorraine Daston calls sumptuary laws an extreme case of rule failure, since they frequently failed to reduce excess and may even have worsened it. English laws seem to have had little effect even though Parliament amended them repeatedly and the Tudors called for stricter enforcement. They were often revisable regulations rather than stable laws, as authorities tried to prohibit the latest rebellious or extravagant fashions.