I know, it doesn't really make sense. But it is because they do this for many other reasons than just the visual display: weddings, ceremonies, opening a business or something else business related. So they often have to do this during the day even if it is sunny, during working hours etc.

According to my wife often it's down to Feng Shui, when you open a business or move house the exact time you move or open etc should be determined by a Feng Shui expert and at this time you let of fireworks, (or more often firecrackers) to scare away evil spirits who may bring bad luck to your new house / business etc.

Because they can't wait, just like they can't queue.  Add to that their stupidity and you have the answer.    

I often wondered the same and my girlfriend doesn't really get it either,

I heard them going off almost every morning. I bought a big roll of them. My GF said I couldn't set them off. not allowed. So as soon as she left I did. The fuse was too short, I never unrolled it and the whole thing pretty much blew up in my face as one big bang. No injury, WYF don't they come with a long fuse?

'Cause it's a laugh.

another good question  why on earth would a grown man or women let kids play with fireworks 

Just to clarify those 'fireworks' we were using while my daughter was present, were firecrackers and hand held mini Catherine wheels, I stood in the middle of the firecrackers (was too slow to get out of the way first time) and no harm came, (a bit smelly and smoky but) and the Catherine wheels wouldn't burn candle wax, we're not talking Monster Hulk Apollo rockets here. Also my daughter was kept back at a relatively safe distance.

Different culture I suppose.  In a lot of western countries fireworks are more of a visual thing, seems like in China there are many reasons for setting off fireworks that have nothing to do with the visuals of a firework.