Why does the tanween letter have an alif at the end

Question
Why does the tanween letter have an alif at the end
Answer
The tanween occurs with all three vowels, the fathah, the dhammah and the kasrah.  There is no alif pronounced when stopping on a word with a dhammah tanween or a kasrah tanween.  If however you are stopping on a word ending with a tanween with a fathah then we stop with an alif.  As in:This is the way Arabs stop on a fathah tanween- with an alif.  If continuing, the alif is not pronounced.
Please see:

http://www.abouttajweed.com/index.php/tajweed-rules/49-the-mudood-the-lengthenings/104-the-mudood-lengthenings-part-3