What is the significance of the "super historic" rainfall for July 2015 in southern California?
This event is "super historic" only because of its proximity to the coast.  It is fairly common to get flash flooding in the mountains and deserts of Socal at least a few times each summer, due to the Southwest Monsoon.  This event was different in that a dying tropical system moving north off of the coast of Mexico interacted with a trough off of the Northern California coast.  The combination of these two essentially funneled tropical moisture north directly into the coastal regions of Southern California.

Linking single events to long term global change is virtually impossible to do reliably, so I won't speculate. However one could argue that rising sea surface temperatures could allow tropical systems to stay more organized near Southern California, and increase the probability of events like this. But again, it is never a good idea to try and link a localized extreme event with a global phenomenon.

In terms of drought relief and reservoir storage these rains have virtually zero impact.  They impacted the southern third of California, while the majority of water storage is in the central and northern part of the state along the Sierras. The rain came down pretty quick and didn't soak in to any of the soils, so vegetation will still be just as dry as before the rain. California needs heavy snows in the Sierras for any serious drought relief. Although these rains have given many people a false sense of security.