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Not doing a write when the disk is idle doesn't save anything even if that write turns out to be unnecessary.

Not doing anything saves power and NAND cycles.

Disk also might be in standby mode, which saves motor spin up and head load cycles too.

The power is negligible compared to the compute. (The disk isn't in stand-by mode If you're doing something that involves transient files.) Moreover, doing the writes now may let you turn the system off. (As long as we're making up situations.)

The goal is optimizing performance as seen by users. Long waits for disk flushes because the system wasted write opportunities is not good performance by any measure.

Nay I'm just refuting the idea that performance is the utmost, singular goal, as is pointed out by vlovich123 4-levels up in the thread https://news.ycombinator.com/item?id=39785690

Modern storage hierarchy has become so convoluted precisely because of many conflicting goals: performance, efficiency, durability, cost, etc.

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