Why use layer masks and adjustment layers?

i've been using pse , cs years.

 

one thing have never understood is: why use layer masks , adjustment layers, instead of creating copy of subject layer (the 1 want make changes to) , experimenting that?  it's quick (ctrl-j), can many times want, i'm not affecting background layer.  if changes, can keep them.  can switch copy on , off compare background layer.  can type of blend or combination desire.  can insert gradient layer(s), select part of copy , (ctrl-j) create new layer containing selected part.  can adjust size, rotate, anything.

 

it seems "layer mask" , "adjustment layer" layer of terminology; can explain (preferably in 50 words or less) how intrinsically different or superior working copies of background layer?  can done them can't done using copies of background layer?

let's start adjustment layers.

adjustment layers layers in layer palette.

but:

real layers made of pixels. each pixel has r,g,b value.

adjustment layers not pixels.

an adjustment layer math formula.

that same math formula applied each of underlying pixel.

what seeing result of math formula. result computed internally , not saved, used display result.

an know ? single formula takes much, less disk space copy of layer.

and can changed easily, whereas modified copy requires being deleted , result totally rebuilt.

now, imagine stacking several adjustment layers: example, black , white conversion.

- hue/sat 1 simulate color filter

- 1 desaturate

- gradient map 1 convert sepia...

 

you can happy because have saved, not original, steps, recipe.

now if open picture @ same time, can move the 3 adjustment layers @ same time apply same editing recipe.

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