When one first initializes a tag wiki, there is no place to include an edit summary. Since tag wiki initializations may end up in the suggested edits queue for review (if the initialization is done by a user without 20k privileges), there should be a field for users to include an edit summary.
I will grant that in most cases, an edit summary is unnecessary for a newly-created tag wiki; however, there are times when additional information about the edit is useful. Consider the following situation: suppose there exists an incorrectly-spelled tag "cmoputer", which has a tag wiki. I might decide to fix this by retagging all "cmoputer" questions with "computer". I might also then decide to copy the wiki for "cmoputer" over to "computer" in order to preserve whatever had already been written. In this case, it is helpful to have a note indicating that the content was transcluded from the "cmoputer" wiki, for reasons of attribution and whatnot. (This is not a hypothetical; we have had to do basically this on Anime.SE on more than one occasion.)