Security??? Why even have it!

I went into the Game Store at Lakeside Mall on Sunday this past week and bought some items. On leaving the store I was stopped at the entrance by a security guard who wanted to see my slip. He took it out, stamped it and pushed it back into my packet within a matter of 3 seconds. There was only one other customer next to me who had the same done to her and we both left. I want to know what is the point of having a security guard at the entrance that stamps one's cash receipt slip. He doesn't check the content of one's packet at all. Stamp and go. I could have had numerous items that i could have ****** in my packet and he would not know any different. I don't get the point of this exercise at all? Please explain it to me! Is it that it was checked if I return an item cause it wasn't?