You split a blob into lines, deduplicated it, filtered it and sorted it. Now the platform you need to paste it into wants a single comma-separated string.
Closing the loop
Joining puts the cleaned list back into the shape the destination expects. Without it, the processing work sits in a format nothing downstream can consume.
Negative keyword lists are the common case
Ad platforms often accept a delimited string. Building the list properly — deduplicated, checked against your actual search terms report — and then joining it produces one paste instead of hundreds of individual entries.
Mind the field limits
Some fields cap the total character count rather than the number of values. If the joined string is rejected, split the list into batches and join each batch separately rather than trimming values arbitrarily.
Try it: Join Lines into a Delimited String on SeoWolf's Notepad.