An internal link coded as [[link ]] will produce an HTML <a> tag that has a trailing space in its output. This is undesirable especially when link text is followed by a non-space character, such as [[link ]], foobar which will have an unnecessary space between the link and the comma. See https://test.wikipedia.org/wiki/User:Huji/sandbox for demonstration.
For the href part of the <a> tag, Parser correctly trims the input (and doesn't try to link to a page titled link_ in the example above) but for the text of the links, i.e the part that comes between the <a> and </a>, the trailing space is not trimmed.
Of note, if the space is in the beginning (e.g. [[ link]]) parser correctly ignores it both in the href and the text of the link.