Finger In Tashahud And Tawatur
True reason behind particular structure of any Ibadah ritual cannot be known with certainty. It is something prescribed by God as He deems fit and only He can do this. The only difference is that some have been made obligatory and transmitted as Sunnat e Sabita (through ijma and tawatur) and others have been optional and are extracted from akhbar-i-ahad.