It most certainly did need the device. Just because it could dodge surface ASW of the time, does not mean it was invincible against airborne ASW. These boats were designed to operate more efficiently under water, which meant running on batteries. Charging on the surface exposed these boats to airborn attack.
The addition of the snorkel was an essential thing had this boat actually made it to service. (which it kinda did, just in the form of the type XXI)
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