He is the weakest juggernaut champion. HP + AP items are not enough for this champion because he has the least tankiness out of all juggernauts and he does not have any built in healing unless he uses his W shield which is on a long CD. He also cannot use any CDR item efficiently so that basically puts his W on a really long CD and he maxes this last unless he starts to build pure defensive items which is third or fourth item.
By giving him healing on his passive, it allows him to have access to healing aside from his W. In the early game this healing is basically non existant but it is there, while in the late game he should be rewarded for sticking on to a target with the passive running.
Keep in mind, the passive healing doesn’t need to heal off minions, it can just be off champion damage only if you like.