How does buying green bags help if the plastic bags are already made?

Change takes time. As demand decreases there will be less manufactured.

It takes a little while to go down the supply and demand chain. When people buy less of a particular product, then the stores order less of it for the next time. If a bunch of stores order less of it then the factories are told by the selling people that they need less of it so they make less. Finally, the research and development people try to find out why, and skew the future products toward what will be demanded more. That's why we have products like "fat free whipped cream topping" which would have made absolutely no sense in previous generations, where the idea would be, if you don't want to eat fat, whipped cream is not a good choice. So finally the manufacturers were all "But wait! We can do it fat free for you! Come back!" I don't know what the plastic bag makers will end up making, but I look forward to seeing it, because it's kind of interesting as well as being more eco.

Green bags are meant for the more people that buy them the less plastic bags stores will need therefor the less bags to make.

Because is everybody starts doing it, then obviously they won't have to make anymore or at least reduce the amount they are making.

Plastic bags are manufactured based on need, so if fewer bags are needed, fewer bags will be produced. The bags are indeed already made but if more people use their own bags, the plastic bags would be used less frequently, therefore reducing the amount of bags being wasted and ending up in landfills. Plus, it is also a way for the store or company to make money because they usually do not charge customers for plastic bags but they do charge at least $1 for the reusable bags.

Supply and demand. If you don't use them those no demand for the next batch to made. It's saving the next generation really.

Because when I use my bags the stores buys fewer bags, when a whole city outlaws plastic bags then all the stores stop buying bags. Eventually the reduced demand means fewer bags will be made. When people use fewer bags then less get released into the environment to become litter. The bag you will need to carry your groceries for Thanksgiving haven't been made yet. So if enough people chose to switch to reusable perhaps it will never get made.

good point; i think the idea is to cut down on the need for them, so they dnt have to make as much plastic

There are a few stores that will not carry plastic bags in the future and you will be forced to purchase the green bags.