As Craig pointed out, space is almost always a premium in pizzerias, especially walk-in/refrigeration space. A pizzeria might have space for one day's worth of balled trayed dough, but not two. With a bulk, they can have two days of fermentation.
Bulks (and mid-fermentation re-balls) have a positive effect on oven spring:the effects of bulk fermentation on a basic dough
I used to subscribe to the yeast/food re-distribution concept that Craig described, but, now, I'm not sure if that's where the improved oven spring comes from. It's proven, without a doubt, that when you punch dough down, the yeast will have access to new nutrients, and this new access will spur on greater activity. But faster acting yeast that reach their target CO2 output in a given time, are, to me, no different than slower acting, nutrient deprived yeast that create the same volume in a longer time frame.
Rather than focus on how much CO2 is generated and how fast it's being generated, I think the real cause for improved oven spring via bulking/re-balling is gluten extensibility. Long fermentation produces gluten by water absorption as well as the extremely slow movement of the dough rising, but, it's all slow enough for the dough to relax considerably. A bulk or a re-ball takes relaxed gluten and extends it further. This extension gives it even greater gas trapping capabilities.
At least, that's my theory. There may be other things going on and/or it might be a combination of things, but, between nutrient re-distribution and gluten extension, I strong feel that gluten extension is doing most of the heavy lifting in the oven spring department.