Convert|Line Breaks -> Wrapping attempts to do the opposite of Convert|Wrapping -> Line Breaks. Email clients and newsreaders, for example, will often limit the maximum line length, inserting additional line breaks into the file that you did not type in by pressing Enter onto the keyboard. This is done because a lot of email and newsgroup software, mostly older software, still cannot handle long lines.
The problem with this is that after the hard line breaks have been inserted, the text becomes difficult to edit. The text will not be nicely and automatically rewrapped when you insert new text into a middle of the paragraph, like EditPad does when word wrap is on. With Convert|Line Breaks -> Wrapping you can tell EditPad Pro to attempt to remove the line breaks that were inserted into the file for the purpose of emulating word wrap. I did say "attempt", because EditPad Pro has no way of detecting whether a line break was automatically inserted to limit the length of the line, or if it was inserted by a person pressing Enter on the keyboard. Technically, there is no difference between the two. But usually, EditPad Pro does a remarkably good job.
If there is no selection when you use this command then it will unwrap the entire file. If there is a selection, only the selected lines are unwrapped. The algorithm that decides whether a line break should be removed or not may make different decisions when unwrapping the whole file versus unwrapping only part of the file. When unwrapping part of the file, it assumes that the selection is your entire document. If you have a file with lines that were pasted in from different files that were hard wrapped at different line lengths, then separately unwrapping blocks of lines wrapped at different line lengths may produces better results than unwrapping everything at once.