Bob Hymes on Lexicon Valley from Booksmart Studios

It’s interesting that we now say “What for?” to mean “Why?,” constructed pretty much like “Wherefore?” On the other hand we don’t say “That for!” for “Therefore” — and “Wherefore” and “Therefore” were part of a regular pattern in which you stuck almost any preposition after “where,” “there,” and “here,” to make the equivalent of “preposition+what?,” “preposition+that,” and “preposition+this.” So, “wherein,” “therein,” and “herein,” or “whereupon,” “thereupon,” and “hereupon.” German still does this with da- or dar- (for prepositions with consonants and vowels respectively), like “darüber,” “davon,” “darin,” etc. (with their corresponding “wo-/wor-” and “hier-“ forms), and frankly I think it’s kind of a shame English has mostly lost the pattern.