Edit: Maybe I don't get the "processing" part... isn't this about creating interactive experiences? The examples demonstrate so and I didn't see examples of processing complex data sets or anything. Would love to hear some elaboration on it.
I noticed the statement "p5.dom lets you interact with HTML5 objects beyond the canvas, including video, audio, webcam, input, and text." This is interesting. Perhaps the point here is P5 manages more interactive components without the developer having to mash different tools together?