Remove line breaks
Hard-wrapped text back into paragraphs. The only real question is what goes where the break was, and joining with nothing is the answer that runs words together. Nothing you type here leaves your browser. It is worked out on your own device, so there is nothing to send and nothing to log — which matters more for text than for most things, because people paste in work they are not supposed to paste anywhere.
What goes where the break was
Removing a line break leaves a gap, and something has to fill it. Most tools fill it with nothing, which welds the last word of one line to the first word of the next:
| Joined with nothing | The quick brownfox jumps over |
|---|---|
| Joined with a space | The quick brown fox jumps over |
A space is right for prose that has been hard-wrapped, which is the usual reason for wanting this. Nothing is right when the breaks were inserted mid-word, which happens in text copied out of a PDF. Neither is right always, so the page asks.
Blank lines are paragraph breaks
In hard-wrapped prose a single break is the wrap and a blank line is a new paragraph. Removing both flattens the whole thing into one block, and the paragraph structure cannot be got back.
So blank lines are kept by default and the wrapping breaks are the ones that go. That is the behaviour that matches what people actually have.