Difference between Zero-Crossing Optoisolator and Regular Optoisolator
Zero-crossing is typically used for incandescent bulbs. You may have noticed that when incandescent bulbs fail they always fail when they're switched on. That's because the mains phase can be near its maximum when switching on. Combined with the low resistance of a cold bulb this results in a high current peak, which may burn the filament. When you switch on a zero crossing you avoid these peaks.  

How I found out? I've known this since my time in college. It simply makes sense.