If you recommend the right shop to a visitor, that shop will gain popularity points. As you accrue popularity points for a shop, its rank will increase, and subsequently, your Join Avenue's rank will increase along with it. Occasionally, gym leaders and other guests will start to show up in your Avenue, and if you recommend them to the right shop, you'll get a massive amount of points.
Keep in mind that fans generated by your game will give a shop less popularity points than a player visitor.
As a shop's rank increases, they'll start offering more and more services for you to buy, and often times at a discount. So it's worth investing some time into. (: