it is just a road that you can build shops that are useless.What is its purpose in the game excacly ?
The shops are not pointless. You can have shops that increase the Happiness of your Pokemon which is usefull when we all find a Riolu in Flocessy Ranch. You can level you Pokemon up and get your Pokemon EV trained.
It is to keep the game fresh. If there was no Join Avenue then the game would be almost identical around Castelia / Nimbassa city.
It encourages the people who play Pokemon to use there Wi-Fi connection along with the wireless connection.
As you can see Join Avenue is not pointless and has many Advantages of its presence.