Why do we need to use a condom?

just wondering what the point of using a condom is if neither me nor my boyfriend have had wih anyone else. neither of us have aids or anything, and have had all our shots. I'm about to get the rod put in (only because i know i'll forget to take the pill), and according to its booklet, there have been no pregnancies recorded while on it. hence the reason i ask why we need to use a condom. i'm not an idiot, i know it's best to use them, i'm just wondering why. thanks :)