The way the function would work: The opacity of specific pixels on the active layer are changed into the brush’s chosen opacity when your brush-stroke touches those specific pixels.
Example 1: A sketched letter J is located on a 100% opacity normal layer with other visuals inside that layer. The pixels of the “J” are at 72% opacity. Your brush is now at 42% opacity and with “replace alpha” blend mode. You draw the letter “J” on the canvas right where the J is. The J now looks lighter because the pixels where you laid down the brush stroke are now converted to 42% opacity.
Example 2: Your boss hands you a messy dirty lineart whose opacity values are all over the place and inconsistent. It’s 23% in this corner, 40% in that corner, 87% in that corner. Yuck. He instructs you to make the values all 50% consistent without messing with the layer properties and without creating or deleting layers. With the blend mode in handy, you set your brush opacity to 50% draw over those lines and get a clean consistent 50% like the boss wanted.
Why I want: This is useful for situations where you need to build up a consistent low-opacity line, shadow shape, or base color on a character with mutiple strokes without having to worry about inconsistent values from the buildup of brush-strokes. It also lets you later come back to the desired area and make clean changes to opacity if you change your mind later.
The problem with regular eraser is that you have to eyeball the value of your brushstrokes until it looks somewhat okay, and you have to redo your shapes if the values end up inconsistent. And adjusting opacities via layer opacity is problematic because it affects the whole layer when you only want to adjust specific areas. And cutting and pasting parts onto new layers just adds extra steps that throws off your workflow.
This blending mode lets you jump right in and make adjustments without having to play around with layer settings or cutting and pasting. The opacity of your desired pixels is adjusted quickly and you continue working on the layer without stopping.
The “Greater” brush blending mode already lets you create consistent low opacity lines. But its limitation is that you can only go up in value. A brush at 40% will not affect pixels on the canvas at 60% with “Greater”. This “Replace alpha” mode lets you work at values both higher and lower than what’s current on the canvas.