Games

Stocks = certificate of ownership in a fraction of a company. The basic principle is sound and goes back at least to the Renaissance, it's everything else around it that sucks and creates a plethora of perverse incentives that benefit the capital owners.

Company stocks are not unique there, it's the most common example but the principle extends to every commodity. You can buy virtual coal or gold right now if you want and sell later, without actually having coal delivered to your doorstep. This is actually a very important market mechanism when it works right because it allows the market to internalize external forces, reducing risk. European energy providers learned this the hard way when prices shot through the roof in 2022 and they were buying gas at "current prices", leading to funds drying up unexpectedly sometimes to the point of bankruptcy, rather than buying gas at "future prices", guaranteeing deliveries that were paid for months in advance. When it works well, speculation is actually an inescapable tool of complex modern economies. Without it you cannot maintain supply chains fit for the modern world, as speculation (when not abused) is the market's way of accounting and preparing for the expected future.

Even in a communist society, you'd need stocks: the disagreement then becomes whether the state should own (part of) the stocks, or if the workers should own all the stocks (legally equivalent to the means of production).