We're fighting heat, and thermal cycling.

At all times, a lower operating temperature is desirable. It reduces errors and failures, and improves device efficiency.

Keeping supplemental cooling on these reduces the range of thermal cycling. Even at lower clock speeds, a fan on the back sink will improve the lifetime of the device. When overclocked but not properly cooled (if the stick didn't die), the chips do suffer and become more prone to throwing errors. In some cases, this persists due to chip damage.

As an aside, I know that the chips in my other stick miners are designed to operate at 40C, so that's where I try to keep them (maybe a touch cooler).