It allows you to use things you would normally not use and it gives a little extra benefit to unlocking these buildings/p2. Also it allows you to vary your playstyle a little. The Gallic, Roman or Ptolemaic team bonusses don't encourage varying your playstyle. The idea with the theatres was also basically a way to research city phase faster. It only affects those who have access to a certain unit, but the Kushite team bonusĀ  also only benefits those factions only with access to elephants.
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