While this was never in the original World of Warcraft game way back when, it was introduced for WoW Classic because Blizzard feared that the hordes of players cramming into early-level zones would effectively make the game unplayable, as competition for resources and quests would be unbearably tough in a single world environment. Splitting up the population into more manageable amounts over different layers was the solution to the problem of overcrowding given the huge amount of people flocking to WoW Classic initially.
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