Pasted text carries padding you cannot see, double spaces after full stops, trailing tabs, indents from a code block. Pick how far the clean-up should go.
Whitespace damage comes in distinct forms and one blunt fix makes some of them worse. Collapsing runs of spaces to a single space repairs the double space that older typing conventions put after a full stop, and the accidental triple space from a bad paste, without touching your line structure. Trimming leading and trailing whitespace fixes indentation carried over from a code block or a quoted email, leaving the inside of each line alone. Flattening tabs to a single space fixes text copied out of a spreadsheet column. Removing all whitespace is the rare case: building a slug, or checking whether two strings match once spacing is ignored.
Most fields do not show you trailing whitespace, but they store it. A trailing space at the end of a display name can push you over a character limit that the interface says you are still under. In a CSV, an unnoticed leading space turns a lookup key into a value that never matches. In a bio, a run of spaces collapses visually in HTML so you cannot see the problem, then reappears the moment the text is exported. Running a clean-up pass before you paste is cheaper than debugging any of that afterwards.
No. The default mode collapses runs of spaces and tabs within each line and trims the ends, but line breaks are preserved. Use the line-break tool if you want those gone too.
Leading and trailing only. It strips the padding around each field without touching the internal spacing that belongs inside a value.
The all-whitespace mode does. The other modes target ordinary spaces and tabs, so a non-breaking space pasted from a word processor may survive them.