One of the most common mappings is to map various operators like x to "_x, always copy to the black-hole-register to prevent them from filling up your register or system clipboard:
1vim.keymap.set("n", "x", '"_x')
2vim.keymap.set("n", "c", '"_c')
But other than x or c, there is also another candidate that regularly fills your register with content you do not actually want there, namely dd: you want to move some text, and after deleting (cutting) some text, you notice that you now have too many blank lines. So you dd the blank line, go to your desired location, press p and notice that you are only inserting a blank line instead of the actual content you wanted to move.
Now you can of course use "1p to paste text you copied before the deletion of the blank line, but this is nevertheless an annoyance. When you dd an empty line, in 99% of the cases, you do not intend to paste that empty line later on. So how about deleting to the black hole register again?
1vim.keymap.set("n", "dd", '"_dd')
While such a mapping does work, it has the rather unindented side effect that all dds are affected, so you also loose the ability to delete (cut) a single line. The solution to this annoyance is to create a conditional mapping: you want to delete to the black hole register when on an empty line, but delete to your regular register when not on an empty line. We can achieve this with the map-argument expr.
1vim.keymap.set("n", "dd", function ()
2 if vim.fn.getline(".") == "" then return '"_dd' end
3 return "dd"
4end, {expr = true})
expr essentially tells vim that the left-hand-sight (3rd argument) should be treated as an expression and not as a function returning some string. That way, expr saves us from the trouble of writing a function that emulates deletion behavior, and let's us nicely solve the issue with only two lines of code.