The correct answer to this open question is the following.
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Much like today, cities competed to increase trade. To draw people to their town, what was placed in the cathedrals were markets next to the cathedrals so people could attend the church services and spend time in the market trading and buying different goods from diverse regions.
This resulted to be a great idea because that was a perfect way to attract enough people to churches because people knew that after the service, they could go to the market to trade, buy, or simply, socialize.
Even to this day, practically in most regions of the world, there is a market close to the cathedral for the exact same reason.