The cost
You see the real size, before you decide.
Most cleanup apps hide file sizes until you're done. Pruune shows you exactly what each photo weighs while you're looking at it — so "keep" and "delete" stop being abstract. You know what the decision is actually worth.
The third option
And then you get a third button.
Some photos are worth keeping but not worth keeping huge. Swipe up to compress and Pruune shrinks the file without touching the memory. Most photos drop 50–70% in size and look identical. You stop choosing between holding on and letting go.