I'm trying to understand how external sorting works from the book Data Structures and Algorithm Analysis by Mark Allen Weiss. The author illustrates an example where he tries to sort 81,94,11,96,12,35,17,99,28,58,41 with a memory which can hold upto M=3 numbers. He uses two input tapes Ta1,Ta2 and two output tapes Tb1,Tb2. Ta1    81,94,11,96,12,35,17,99,28,58,41Ta2Tb1Tb2 In the first step M numbers are read into memory then, sorted and written to Tb1 and Tb2 alternatively.Ta1    Ta2Tb1    11,81,94   17,28,99   15Tb2    12,35,96   41,58,75I understand how the algorithm works but, don't understand why it is important to write results alternatively. Couldn't this be achieved without alternating?
importance of alternate storage in external sort
algorithms;sorting