system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How and when did we realize stock trading was something we needed? What prompted the first companies to issue stock for outsiders to buy shares in their company?

Since the whole sector from the outside looks really silly (it isn't), how did the first few pioneers in the sector realize there was a need for it?
gpt
It was a long time ago (think like the 1700s) and it was necessary to fund growth and to create bigger opportunities.

To start, let's understand what came before.  If you were a merchant in the Middle Ages you were basically trading on your family's money.  You paid for the goods, you paid for the ship to transfer them, and if that ship sunk you were in deep trouble.  You were *personally liable* for the debts of your business.  Even a group of people funding an expedition would be in the same boat (heh) in regards to what happened.  

Obviously this means that early companies tended to be a single very wealthy family and relatively limited ventures anyway.  You simply couldn't fund the kind of large scale companies that you see today.  There wasn't enough money in one or a few people's hands, and it was risky.   A single bad deal could break an entire family.

Jumping forward a bit, as business ventures got larger and more complex (because empire) people came up with an interesting idea.  Instead of paying for an expedition to the Americas or to sell wool in Flanders or whatever by yourself... why not crowd fund the thing?  So you would create a company, and sell ownership stakes in it to your wealthy family, friends and fools.  This gave you a source of funds to make it happen without having to fund it all yourself.  Everyone risked a little bit and would get paid based on their stake at the end.    Of course there was still some issues with the debts of the company falling back on the owners... but this was fixed with the notion of "limited liability" which basically means that as a owner of a company you are only responsible for YOUR share of the company and if that hits zero that is the limit of what you can lose.

That developed into our modern system where shares of ownership in massive companies are traded at the rate of millions a day.