Very similar to his role in Boys Before Flowers, Kim Joon is the one who binds the F4 together. In the drama, Joon plays Song Woo-bin who's usually uninvolved romantically but provides his friends support and a listening ear to their problems. In real life, Kim Joon also acts like a "big brother" (and in actual fact he is the oldest among the four) and shows support to his F3 friend's projects.
Kim Joon and T-MAX showed up to support Lee Min Ho in his recent Cass Beer club event and even stood on stage with Min-ho to promote a healthy responsible drinking habit. Joon also congratulated Kim Bum on his new projects and attended a fan meeting with the youngest of the F4 members.
Even with their busy schedules, F4 reunited (not so sure about Kim Hyun-joong though) to work on the Special Edition music drama. To work on this post-drama project, it was reported that Kim Joon had a hand in rounding the boys up. He personally contacted them. With that, Joon played an important role in making the special edition project a success. Since it's launch on Mnet, the music videos have been viewed by a whopping 11 million viewers.
Some might say he isn't as popular as the other three guys and thus has more free time but perhaps he's just a regular nice guy and values his friends.