Removes spaces and newlines from the start and end.
Not needed for a named parameter (neither in the template call, nor in the template code). In a template call, even for an unnamed parameter it is usually easier to write it as a named parameter, by putting e.g. "1=" in front. Note however that this affects the numbering of the remaining unnamed parameters.
Also not needed for a parameter of a parser function, it is trimmed anyway.
Even in cases where spaces do not affect rendering, trimming can be useful in substitution to avoid clutter in the resulting wikitext.