This collapses a multi-line list into a single line with a delimiter between values — the reverse of splitting.
Why the reverse trip is needed
Lists are convenient for processing and inconvenient for input. Many fields, query parameters and configuration values want one string with separators, not many lines.
Choose the delimiter for the destination
Commas for a spreadsheet cell or an IN clause, pipes for systems where values might contain commas, spaces for a command line.
The round trip
Split, process, join. That sequence covers a large share of everyday list work: break the blob apart, clean it, put it back together.
Try it: Join Lines into a Delimited String on SeoWolf's Notepad.