When you love somebody very much, you think about them even when they are not there. This is ’shalach manos’; sending gifts of food to one another, through a messenger. Why not face to face? Because in this way you are telling your friends that you are sending them your love, your thoughts, even when they are not there.
Everybody knows that the downfall of the world is ‘loshon harah’. The Gemara says that Haman is the master of ‘loshon hara’. Mordechai and Ester are masters of NON-’loshon hara’. On Purim we do not send ’shalach manos’ face to face; we are telling each other, even behind your back, I shall not speak evil about you.