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> Preventing Ottomans from emerging serves the main purpose of keeping Balkans christian, while giving more breathing time and for the region to avoid becoming basket case it is today. But really, there's all there is to it: keeping it christian.
I think there is more to it than that. Without Ottoman rule the Balkan countries probably would have been more ethnically homogenous, without having ethnic groups widely scattered across the region as in OTL. The different countries might still go to war against each other now and then, but there wouldn't be as much domestic strife and separatism. With greater stability can come greater economic development, especially given their proximity to wealthy countries nearby.