Sample all layers is a sort of 'preview' tool.
If you have multiple layers with various colours on, and you want to see how they would look if you say, 'smudged' them together if they were on one layer, create a blank layer above all your colours, tick sample, then smudge over on the Sample layer you made. You will notice it gives you a 'sample' of how the smudge would look of all the layers below it.
You can then hide this layer or make it visible and you will notice your previous layers are untouched, in a sense it sort of duplicates whats below it with whatever effect you use, onto the new layer
![Smile :)](./images/smilies/icon_e_smile.gif)
It's hand if you want to try out colour combos or shade without losing your base colours.