stun server. (just want to clear my concept)

i know this is not a right place to ask this question but it is just a concept clearing query. if appreciate any comment but since i only use debian as a whole solution i could not stop my self throwing this question. please ignore if you don't like to answer. actually i am looking in to

Re: stun server. (just want to clear my concept)

stun server is mainly used for Voice and Video due to their UDP in nature. so that if UDP recipient want to response back with voice or video packet it returns back to Stun and stun route back to our local server. but the question is why?. if i have a public Ip which is published behind