Their output is better. The solutions that they select are more robust, creative and scalable. The medium-senior developer understands the problems, and probably, knows how to avoid traps and how to minimize the number of bugs. Besides, these developers code is faster than junior developers, and they work independently.
The Risky Game of Relying on a Junior Developers Team
Dor Moshe

This is all 100% subjective. There is no evidence to back up this claim at all.

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