Help understanding the expansion of Italian City-States during the early renaissance.

I’m working on finishing up an AP European History summer project and I’ve come across a part which confuses me.

This is, the growth of Italian City-States during the endemic warfare between pro-papal and pro-imperial factions. The two factions used a strategy of weakening one another as opposed to fighting with full force tactics. The textbook describes this strengthening the merchant oligarchies of the cities because the Italian cities were free to expand on their own.

My question is, why would this happen? Why would the warring of these factions create an environment in which the merchants would thrive?