Answer: In my opinion, there was no granary in Harappa or Mohenjo daro. These site were Necropolis (Burial Sites) and what was stored in such granary type buildings were only mummified dead bodies in earthen pots. See my web site for more details.
Or
The answer is that the existence of a granary shows that there was surplus food production in Indus valley civilization. The second inference that could be drawn from this fact is that there was organized administration like bureaucracy or like priest king , who could order and organize collection of grains and storage for long time use.