Guilds played a leading role in the economic lives of medieval cities because they exerted a great degree of control over the entire process of making and selling various goods and services. There were guilds for essentially every kind of economic activity. In some cities, guilds were so prevalent that there were even guilds for prostitutes.
Guilds regulated the production of goods and services. They set standards for the quality of goods that had to be met by all guild members. They sometimes specified the processes by which the goods would be made. Only people who belonged to the guild could produce the product that guild controlled. The guild therefore also controlled who could work in a city. They set up procedures for apprenticeships and for determining when a person was deemed to be good enough to set up shop on his or her own. They often even set the prices that could be charged and otherwise regulated competition so that all members of the guild could make a living.
By doing all of these things, guilds exerted a tremendous amount of influence. They were like labor unions and government regulators all rolled into one.